Nina, A few ideas:
1. Sometimes there are mineral deposits (from 'hard' water) on the surfaces the gasket resides between (for instance, around the bottom of the float tube that sinks down onto the top of the seal). You can try carefully sanding them off with fine sandpaper.
2. If you have very cold water, especially, in winter months, a seal will be tested! Look for a new seal that is shiny on its surface. This indicates it may be silicone-based (not rubber) and they work better in areas with very cold water.
3. When installed normally, distortion of the seal is not a problem. You may just need to reinstall to make sure everything goes smoothly. But, first, gently pull the seal on opposite sides, with both hands. Do this moving around the seal to be sure it is properly seated in place. If nothing else woks, time to call a professional : )
Close your eyes. Attaching something, the turning motion is (usually) clockwise: Righty-tighty; lefty-loosey.
You "think" it would have been more difficult? No disassembly (and reassembly) required on this one.