Brian, In the base of the toilet there is a little hole if this hole is blocked (due to hard water) this will cause a toilet to flush sometimes and not others.. Flush your toilet and when the flapper closes dump some red food coloring in your tank, when the tank is full flush it. You must watch real fast here!! See if the die comes out of that little jet first.. If it does not then this is your problem !! Easy fix just install a new toilet or see if you can unblock the little opening.. Hope this helps..