Fluidmaster flappers are just fine. I'm not sure what you mean by
"getting air in the flapper." Here are a couple of ideas.
First of all, sometimes if the water level is too high in the tank,
the flapper can get sucked back into place prematurely. You can try
lowering the level to see if it helps.
Secondly, what type of fluidmaster flapper is it. Is it just the
universal replacement, or did it come with a flush seat that you
adhered to the flush valve?
Did you simply slip the rubber ring over the overflow tube, or did
you actually attach the flapper to the pins on the side of the flush
For being such simple technology, there are many details.