Whats the trick with the thump? I have repeatedly pushed the valve stem back into the stem and try after try am failing at thumping the valve stem out to where I can grab it.

I have pushed it through with a piece of bailing wire; each time blowing out all excess slime then will line it back up, thump it 2,3,4 times and it will not come back out. How, in every video I watch, is it only taking two thumps for the valve stem to come back out?
I'm going out on a limb and suspect you have removed the end nut and have pushed the valve core down into the tube to allow the sealant liquid room to move past and into the tube ?   

Short answer is, don't do that.

Use tubes that have a removable valve core.  These can be identified by two flats on opposite sides.



Since you've probably already got the valve stem in the tube, your only options are to manipulate the tube to get the core back into the valve, or to cut a hole in the tube opposite the valve for easier access.  This last suggestion is the last resort, cos the sealant will make it hard to put a patch on the tube, so clean it really well before buffing.