Toto Ultramax question

I LOVE these things. It is my fourth and final toilet but this time it seems to have an issue.No signs of leaks but left overnight or for long periods on non-use,it does not flush. Yesterday I plunged it thinking it was clogged. It was not. It just seems to lose force. It did the same thing today and I flushed once to test it. It just sort of did nothing and then on the second attempt it did the job perfectly as ususal.
It is in a bathroom that gets little or no use.
What might be the cause?
Again,no visible signs of leakage from below.No noise.No dripping.
Thanks in advance.

Before you flush it the next time, pop the lid to the tank and check the height of the water there...once in awhile, a flappper is defective, the fill level isn't right, or the refill hose isn't positioned properly. On some when new, I've noticed the fill valve is a little sticky, so it may not actually run after a flush.