Gerber help

I have a Gerber toilet with flapper problems. The old flapper was not seating consistently, so I replaced it with a Korkey. This flapper immediately shuts when the the lever is released, so in essence, you have to hold the lever all the way down (to keep the flapper open) until the flush is completed.
Is there some adjustment I'm missing? I've thought about replacing the entire valve and other parts but am confused about that as well. This toilet has a circular baffle around the flapper post. Any ideas?

It sounds like the flapper chain is mis-adjusted. The chain should be taut enough to pull the flapper all the way up, but with just enough slack to let it close all the way back down to seat properly.

It's also possible that your lever is not allowing the flapper to open all the way. If that's the case, you'll probably need a new lever.