Your initial memory could be during the summer time when it takes less hot water to make a shower. Or fewer teenagers, lol. A cracked tube, dead element, and full of sediment are other things to check.

I upgraded my 50 gal tank wjth a 105 gallon Marathon years ago. Cranked up the temp a bit and installed regulators on the showers to prevent excessively hot water coming out. Even with a family of six, we never run out of water.