I have had this problem myself, usually when I was printing on a flash plate, or printing on ink... when you push the ink through the screen onto the fabric, the ink can embed itself into the shirt. when you push the ink through the screen on top of ink, the ink has nowhere to go except out to the side...(not to mention that after this occurs you have ink on the back of your screen and have to clean the screen before continuing).
I started using higher mesh screens when I print on a flash plate, stopped flooding the screen and don't apply so much pressure to the print. It's more of a personal thing... you need to just practice with it and build your own technique..