Mansfield 35 problem

I have three Mansfield 35 toilets in my home the two upstairs toilets seem to have the same problem. When you go to flush them it takes a great deal of force to lift the "plunget" inside that releases the water into the bowl. I have replaced the levers on both toilets twice. It feels like the plunger is suctioned down very tight. Any ideas on how to resolve this?
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