Is the unit in an 'unheated space', like a basement? If so, consider wrapping it. (Read on before you get too excited ...) And is it a lot colder in the winter down there than in the summer? By how many degrees between the seasons? Also, are your hot pipes 'insulated'? (There is 'tubing' you can buy at the Home Centers for pennies ...) Also, where's the heater in relation to the bathrooms having the problems? (In my house, the hot water heater is, basically, directly below each of the bathrooms. In other words, the tank isn't on the OTHER side of the house where the hot water would have a loooong distance to travel before it gets to the bathroom. This is where you might have heat loss.) In case someone else needs to know, what is the make and model of the unit? Do your pipes travel along outside walls?
There may be other questions but I can't think of them. Perhaps others will 'ask'. In the meantime, post back w/answers to the ones above. For now, my best to ya and hope this helps.
PS: We have a 40 gallon natural gas HWH with a power vent. It's in an unheated, below-ground basement in corner with 2 outside walls. It's not wrapped although the HOT water pipes are wrapped. We have no problem w/hot water and we, too, take what we consider 'normal length' showers.