It may not be your problem but you water heater is of the age where many had failed dip tubes. They dissolved and broke up. Sometimes there would be pieces of plastic in faucet strainers, etc. They were recalled and if yours is bad, then they will pay for a plumber to replace them. You should contact your plumber and the water heater company about that. The dip tube is a plastic tube hooked onto the cold water inlet at the top that channels the cold water down to the bottom for heating. When it breaks off, the cold water don’t go down and consequently doesn’t get heated like it should. If this happened, you would notice a quick shortage of water. Since you should get multiple showers out of a 40 gallon heater and you indicate that you don’t get one full shower in the winter, then I would look into the diptube. Check the manufacturer’s website for diptube info. It wouldn’t be a seasonal problem though. If not the diptube and since I can’t think of anything else, then jb has the only other reasonable answer to your problem as far as I know.