I can't speak to the maintenance of your specific unit, I'll bet others here can. If it hasn't been blown down any, anode has not been checked, and no cleaning has been done of the central chimney then it could be losing a lot of capacity and have a lot of sediment/corrosion products inside. There might be a pile of scale fallen on top of the burner too. Seems worthwhile to have someone do maintenance on it with a back up plan of replacing if it isn't repairable.
There are several workarounds you can try that will make it behave as if it had some additional capacity:
1. Increase the water temperature a few degrees. What are you running at presently? Don't go to excessively high temps especially if there are small children and no tempering system, but you probably have some room on this.
2. Try some of the newer low flow showerheads. You will probably have to order them online as the big box stores never seem to have anything worthwhile of that nature. I'm using a Roadrunner and two HighSierra's for my showers. At 1.5-1.6 gpm compared to 2.5 gpm for the standard "low flow" showerhead (not really low flow since they have been the standard for a long time), it is like having another 60% capacity or so. Plus, it will save you money on water, sewer, and gas and pay for itself in less than a year typically.
3. Insulate any hot water lines you can reach. This will reduce ambient losses and give you an extra degree or two at the tap. The pipe insulation is cheap and easy DIY...IF the piping is exposed or you can find access in a fashion to slide the insulation down the line between rafters, etc.
4. Consider insulating the tank itself with a wrap around jacket (these are available in the big box stores.) This might not give you any noticeable effective capacity increase, but it will reduce your storage heat losses, meaning the burner fires less frequently just to keep the tank hot. And realistically, if the temp is falling close to the burner firing temp now, if you cut the losses by 1/2 or 1/3 then the starting temp of the water will be higher for you.
In the past year I've done all of the above (except for increasing water temp.), and replaced the old top loading washer with a front loader. My water heating gas use is half of what it was this time last year...and ~30% of that remainder is storage losses.