What brand, how big, what temperature setting before and now after you changed it, and gas or electric? Did turning up the temp fix it? How many and where do you have any single handle faucets and are any of them auto temp control? How old are your faucets? Have you checked the temperatures when working right and when not with a hot tub or oven thermometer and what are the temps as checked and as listed on the thermostat? Also do thermometer checks under any temp control or single handle faucets when working right and when not. When your hot tub isn't getting enough hot water, go to another faucet somewhere else in the house and see if you can still get hot water and let me know if it does.
I have some possible ideas based on assumptions but one/some of our readers don't understand when, why, and how we have to make assumptions to answer a question so I guess we can do it the hard way.