Where do you live and what is the temperature of the cold water coming in?
A tankless system can only raise the temperature a certain amount, and in the winter, the incoming water can get close to freezing. In the summer, it can be 40-50 degrees warmer, so it can produce quite hot water then.
The things get calcium deposits in them too, which restricts their ability to raise the temperature. It might just need a good cleaning. You'd have to refer to the manual on how to do that. It could be it is fine and doing the best it can.
Tankless systems aren't for everyone, everywhere. You may need to learn to live with their limitations. Some can support a second one in series to raise the temperature more, or yours could just be undersized.
Use the search function and you'll get lots of reading material as this has been discussed fairly frequently here.