A few things come to mind:
- Most valves these days have a hot limit adjustment. In the winter, the cold water can be 40-50 degrees or more colder than in the summertime. You may have to readjust the hot limit.
- the dip tube in the WH may have broken. This was a problem with older WH, but could be a factor. This tube directs the cold water towards the bottom of the tank so the hot near the outlet (the top) can supply your needs. If the tube breaks apart, the cold mixes directly with the hot and the temperature quickly drops as water is used.
- the thermostat may not be working properly, preventing it from working properly (true for both gas and electric)
A thermostatically controlled valve is more expensive than a pressure balanced valve (probably what you have). Assuming there is hot water, it does a better job of keeping the temperature constant than a pressure balanced valve. It isn't magic, it can't make hot water, but just modulates it automatically if it is there.