Yeah, I have had the same problem with my Bemis 500EC 146 Toilet seat. The toilet seat tighten screws just doesn't work as well as my old Bemis toilet seat, that had a screw on the top, instead of the new hand tighten lock.
Essentially, what I've done is make sure you have the two locks on top, unlocked. Tighten the screws on the bottom, while putting weight with your other hand on top of the lock, so that it's flush with the toilet bowl. Make sure to hand tighten super tight. Once both screws are tightened down, lock the top two locks (but make sure you really push them hard to the right so they are locked. These seem to loosen up and come unlocked if you don't.
If that doesn't work, then consider getting Toilet seat sticky washers for the screws. This might help hold the screws in place better, than just hand tightening. My older Bemis toilet seat actually had sticky pads on the top of the toilet bowl (between the toilet and the toilet seat hinge, which may have helped the screws stay tight for 20 years.