I still use MaxoPake inks from Union. They are thick but they are good inks. Once the ink is moved around a bit, it loosens. If you thin the ink, you will have a problem with the opacity.
If you stir the ink up really good, it will loosen it up.
I'll reiterate what some have already stated:
Use a spray adhesive. Obviously not too much, but use enough so when you are printing, the shirt doesn't pull up.
If you are using MaxoPake inks, my guess is you are printing then flashing the ink, then printing again. This will cause the platen to heat up and every time you pull your screen down, you are helping ink to stick in the screen. To remedy this, if you are a medium speed printer, I would place a shirt on the platen, rotate it under the heat (getting rid of any shrinkage), place a shirt on the next platen and so on until the first shirt is back to you. Print that one and rotate it under the heat. Flashing the print is all you need. You do not want to cure the ink at this point. I believe the flashing point is a little over 200 depending on your ink. If you cure the ink before you print again on top of it, you could get some flaking. Anyway, by the time you rotate the first print back to you, it will be cool enough to print on. Remove that shirt and place a new one on and rotate it under the heat while you print, remove and place a new shirt on that one. By the time you are finished, the original shirt will get back to you already preshrunk.
Another factor could be your squeegee. If it's way too soft, it will push more ink through or if you are using too much of an angle. I would use a medium to firm durometer squeegee and use about a 70 degree angle. Obviously you don't want it straight up and down, but too far down will push too much ink through your screen.
Hopefully this helps.