If you use self-made beef stock, try boiling a bit longer, so that the flavour of the beef is a bit more concentrated.
Boil your soup a bit longer.
Use more beef stock and/or cubed beef.
I don't know how much herbs you put in.
Maybe a bit more pepper or rosemary can make a difference.
cooking.stackexchange.com/users/4580/Mien
One way to boost flavour would be to roast some beef bones till they brown a little (you can get bones from your butcher) and then cook them with your soup.
You can take them out when the broth is cooked.
This will add a nice depth of natural 'beefy' flavour.
cooking.stackexchange.com/users/5449/Red Spatula
Reconstitute and chop up an ounce of dried porcini mushrooms, reserving the liquid. Filter the liquid to remove grit, and then add it and the mushrooms.
You'll add a good burst of umami and a nice earthiness, without adding too much bulk.
Also, fine-chopped celery works to give some interesting higher notes.
cooking.stackexchange.com/users/3819/Dave Griffith
I have no idea how this works
but... I find with a lot of beef soups/stews letting it go cold and re heating it the next day seems to do the trick.
Some how all the flavor comes out of the meat when you cook it and then goes back in long after my guests have left.
If any one has an explanation for this please let me know!
cooking.stackexchange.com/users/2391/vwiggins
I am making my version of Beef Barley soup and I add cumin,curry and tumeric as I am browning my diced meat and onion.
I only add a small amount of each
but it adds a lot of flavor
and I use vegetable juice instead of boulion.
cooking.stackexchange.com/users/8744/Connie
It sounds like you had enough spices.
Are you sure you used meat meant for soup?
Cuts intended for frying/roasting are more tender, but have much less aroma.
For soup, you need older, tougher meat.
Also, you say "Cubed beef", which sounds as if the bones have been removed.
Cooking the meat together with the bones gives you more flavour.
Third, you are supposed to sear the meat and sweat the onions before simmering, because that develops flavour.
It doesn't get clear from your question if you did that.
Another way to have more flavour is to use more fat.
Even if the fat itself is neutral tasting, it makes you taste the other aromas stronger.
Add it when sweating the onions.
It may be worth trying to first roast the barley in the oven, or even prefrying it, like prefrying rice for a Balkan/Middle Eastern dish.
I haven't tried this and cannot guarantee that it will work.
You can also use MSG or other glutamates.
They are usually not sold pure in the supermarkets, but are included in other products, like broth cubes and seasoning powders.
Maybe you can get them pure in a Chinese shop.
None of the ideas above apply to your already cooked batch soup (you could take out some of the liquid, dissolve a broth cube, and put it back, but it will make it much saltier).
The only things I can think of will add flavour, not strengthen it, thus changing the original idea.
I don't know if this is a problem for you.
If you don't mind changing the flavour, you can 1) add other herbs (they need to be fresh, dried ones should have cooked a bit).
Parsley, marjoram, and lovage come to mind.
2) Add condiments.
Worceshire sauce is unusual in soup, but works well for my taste.
3)
Cook chopped dried mushrooms in a very small amount of liquid, then add them to the soup together with the liquid. 4) throw in crumbled feta cheese (good quality, preferably from sheep milk).
Other ideas which change the flavour, but cannot be applied to the already cooked soup, would be adding celery and bay leaf.
cooking.stackexchange.com/users/4638/rumtscho
I've had good luck adding a small packet of unflavoured gelatin, it's not enough to gel the soup (gross), but it adds some of the body that long simmered beef bones would add.
Celery is essential, as someone else mentioned.
You wouldn't think it would add much, but there's something magic about it.
I also toss in some celery seeds (not celery salt!)
I grind them to a fine powder for beef barley soup but add them whole for chicken soup.
cooking.stackexchange.com/users/6083/Tungsten
I too have trouble sometimes when making soups.
I recently discovered that about a half of a medium rutabaga peeled and diced in 2 quarts of soup adds a lot of flavor.
It takes about an hour to cook them so plan to put them in at the beginning.
You can also add a pinch of dried thyme.
Be cautious with thyme because it can easily overpower your soup.
cooking.stackexchange.com/users/21796/Rose
Yes, with the beef stock add same amount of chicken stock, and a touch of sirachi sauce
cooking.stackexchange.com/users/35884/dan v
This time around my beef barley soup is tasting a bit on the plain side. It has the right amount of saltiness but is lacking in the flavorful department. The ingredients used thus far have been:  Beef Stock, Water, Salt, Ground Black Pepper, Seasoned Pepper, Rosemary, Onions, Carrots, Barley, and Cubed Beef.  What can I do to help boost its flavor?
What can I do to add more flavor into my beef barley soup?
cooking.stackexchange.com
<beef><spices><soup><broth><barley>
cooking.stackexchange.com/questions/13492
cooking.stackexchange.com/users/3478/duchessofstokesay
Makr sure you sear the beef first to release flavour, and try letting it get cold and heating it up the next day. Adding more beef or chicken stock, cubed beef sriracha sauce, pepper, rosemary, dried mushrooms, feta cheese, celery and bay leaf or half a rutabaga can help. Try using more stock or a mixture of stocks. Add some mushrooms or some rutabaga.
Try using more beef stock or add some chicken stock with it. Add more pepper or rosemary. Add some mushrooms, some rutabaga or sweat the onions.