I had O'reilly's check out battery and all cells are fine, good starting point.Just my $0.02 here....
Exactly how old is the battery?
A battery could indicate 12V when checked with a meter, etc. but if just one of it's cells is bad or going bad that would be one indication that the mower will not start or having a hard time starting?
If battery is not that old I would replace it with a new one and see if that corrects the problem?
If it starts your mower then great your back in business....if not then still good that you know for a fact you have a new battery.
I experienced this once.