I have 3 toilets in my house and two of them have the same problem. You have to push the lever very hard to flush. The flapper is sticking in the closed position. I have adjusted the length of the chain to too tight and too loose and it doesn't make a difference. I have cleaned off the flapper and the mating pipe but this makes no difference. Once the flapper opens flushing works just fine. The tank water lever is correct. I have a clorox-type cleaning thingy in each tank but the problem was occurring before I put that in. I'm not sure what to try next.