Any of the things Jay J mentioned could be it, but I have another pet theory that I bet is a contributor. I notice that I use much less hot water in the shower in the summer than in the winter. It feels the same, but ones sense of heat is different in the summer vs. winter. I have noticed that my valve points at "one o'clock" in the summer (cold being to the right and hot to the left) and that it is pointing at "11 o'clock" in the winter. If you have an undersized unit (for demand) then this might account for some of the seasonal differences. I'm not a plumber, I'm just saying that people use more hot water in the shower in the winter and there are a lot of reasons for this, not just the ones I mentioned.
I would look at what Jay said, maybe also turn up the water heater temp in the winter until you can fix it or get a better unit.
Do you drain the crud out of the bottom of the tank regularly? This can reduce the efficiency of your unit.