Should have nothing to do with mounting the toilet. There is a rubber seal on the bottom of the bowl that rubs against the flapper to seal it. Hold the flap open and run a toilet brush around the bottom of the seal. Then boil up some water and pour it into the toilet. Let it sit for awhile. Next after that has cooled a bit, run your fingers around the seal and press it into place against the flapper to re-seat it. If you don't wear gloves for this don't lick your fingers when done...I got this info from the manual for my toilet, well most of it. It has worked every time...
2015 Chev Silverado 3500 dually D/A
2016 Cedar Creek 36CK
B&W Patriot 18K, Trail-Aire pinbox,
Level up, dual panes & all that stuff...