Depending on how much hot water you get before it cools this might indicate a dip tube problem (too short? broken?)
Time of year might be the issue...colder weather means colder incoming water. (Where I live the supply water temp has dropped from about 68 F late summer into the 50 F range, and it will continue to drop throughout Jan. and Feb.) This will result in two things: the mix at the shower valve will require more hot water than it would have a month or two ago to acheive the same shower temperature, and the colder feed into the water heater will also result in a more rapid drop off in temp. As a result the same shower in my locale uses about 50% more hotwater in winter than it does in summer.
Did you ever measure the hotwater temp of the old heater? Have you measured it with the new one? My in-laws' 18 year old (or something like that) water heater seems to run hot as hades and the hot water in it lasts seemingly forever. I'm careful of that one with the kids because of its scald potential. When they replace it and set the dial to the default setting of 120-125 F they won't have nearly as much apparent capacity I suppose. (I realize you have yours cranked already...but I wonder if the old one went higher?)