When you are printing and flashing you need to make sure that you are letting your pallets cool down prior to printing the next color.
On Rotary presses like the Riley Hopkins it is good practice to set the flash up at the 1st station after your print station. This allows the pallets to have 2 cool station prior to printing again.
If you have a single or 2 color press be sure to give the image a few seconds to cool prior to printing the second hit or next color. The heat generated from the pallet will actually cause the screen to stick to the hot ink.
When you pull the screen up you will see the screen release from the outside edges in. Once it gets to the middle it will "pop" off the screen leaving a ring in the middle of the ink. To avoid this let your ink cool and be careful of your flash times. The longer the run the shorter the flash times can be as the platen retains heat. Heat on both sides means shorter flash times and shorter cool down times.