Cooking anything really lean for a long time can result in super dry meat.
Chicken breasts have little fat and little connective issue.
The same goes for a lean beef roast.
You can do chicken breasts in a crock pot - just not all day, more like a few hours on low.
Try something with more fat like a thigh (or any dark meat) or a beef chuck
(shoulder) roast
and you should have more luck.
cooking.stackexchange.com/users/1374/rfusca
I know I'm weighing in on an old question, but in my experience there are two things meat needs to do really well in the slow cooker on a long cycle (ie all day): the right amount of fat, and plenty of collagen.
We all know about fat keeping meat moist, that's true of other cooking methods, and as others have said, chicken breast does not do well cooked like this as it is too lean.
But the collagen is a lesser known issue.
In my opinion it is even more important.
We hear a lot about how you can cook cheap tough cuts in the slow cooker, but what many don't realise is they are actually better than tender cuts on a long cooking cycle.
The collagen breaks down to gelatin which lengthens the cooking process and gives a moister, juicier result.
IMO, the reason chicken thighs do better than breasts in the slow cooker has more to do with collagen than fat - they'll still do well even with the fat trimmed off.
So you need not just meat
that's a bit more fatty, but a lot tougher.
Use legs instead of breasts, chuck instead of lean tender beef roast, pork or Lamb shoulder instead of leg or loin.
To really see the magic, use cuts that are usually impossibly tough - beef shin or short ribs, Lamb flank, oxtail, Turkey drumsticks.
If you want to cook tender, lean meat in the slow cooker, reckon on a much shorter cooking time, like 2-4 hours.
Exact time depends on the individual slow cooker so be ready to experiment.
cooking.stackexchange.com/users/27505/hippytea
I had this problem for a while as well - it turns out my slow cooker was cooking at too high of a temperature (even at low), which would create problems, not only with my chicken, but other things as well (pasta sauces would boil and burn, etc).
Reviews on Amazon revealed that this is a fairly common problem with certain brands of slow cookers (especially those at lower price points).
Once I learned this, I got a nicer slow cooker with a programmable probe thermometer built in, so it will switch to "warm" mode once my food hits the optimal temperature.
This has helped keep my meat moist and my sauces unburnt.
cooking.stackexchange.com/users/9192/Steggo
It's hard to ruin anything in a crockpot (slow cooker)
Does your crockpot have a thermostat?
It may be faulty
cooking.stackexchange.com/users/3203/TFD
Yep I found out the hard way that slow cooking boneless chicken usually dries it out.
It is not a "tough meat".
Speaking of roast beef though, I got a roast from my mother-in-law and it sat in the freezer for a couple of months
so I decided it needed to be cooked.
I had never cooked a roast before.
I found a recipe online using a slow cooking method for 4 hours (dutch oven pot @275F).
Because of the type of cut the roast was (rib roast)
it was so tender it melted in your mouth.
I tried the recipe again about a month later but bought "outside round" roast instead of rib roast (or chuck, shank, brisket etc.)
and it ended up slightly dry.
Why?
it doesn't contain as much collagen as the other mentioned cuts.
The collagen is the key to melt in your mouth texture and robust flavor for slow roasting.
For pork, try a pork shoulder or butt not a tenderloin if you are slow cooking.
The results will be amazing!
cooking.stackexchange.com/users/68975/user68975
I would cook it in a cream of vegetable or chicken soup if you want the easy short answer.
Even chicken breasts turn out tender and moist for me this way.
cooking.stackexchange.com/users/45680/user45680
Bottom line.
No chicken breast in crockpot.
The meat must be fatty, tough, to do better in a crockpot.
I will never cook a breast again in a crockpot.
Once I thought cornish hens would do well..
no, it was horrible.
I just purchased a pressure cooker!
cooking.stackexchange.com/users/49546/user49546
I crock a lot.
Even with juice
any meat
you over cook will dry out.
I have switched to thighs and I remove the skin and fat.
Cheaper
and it crocks a lot better for me.
You do end up with the bone to remove.
cooking.stackexchange.com/users/45636/paparazzo
Just cooked some chicken breasts today. All day in the crockpot and when done the meat was horribly dry.  I put chopped onions, carrots, spices, and coconut milk and 1/2 cup water and 4 chicken breasts.  There was plenty of fluid there when it was done, but the cooked chicken meat was really dry.  This seems to happen with beef roast as well.  Is there some trick to getting meat to be tender when cooking in the crockpot?
Why does meat in the crockpot always end up dry?
cooking.stackexchange.com
<chicken><crockpot>
cooking.stackexchange.com/questions/21511
cooking.stackexchange.com/users/9209/Nerdtron
Cooking lean meat like chicken breasts for a long time, especially at too hot a temperature, can result in very dry meat. Try using chicken  legs instead of breasts, chuck instead of lean tender beef roast, pork or Lamb shoulder instead of leg or loin. Check the temperature of your crockpot and avoid cooking lean meat for a long time.
Lean meat does not do well in a slow cooker. You should use fatty, tough meat. Check the temperature of your slow cooker.