Does it move at all, or just won't go towards the hot side? If it freely moves, but only towards cold, it's almost certainly the max temp setting. Now, how to adjust that will depend, it could be a screw, a lever, a cam, etc. Generally, to adjust it, you need to remove more of the trim.
So, if it moves, but stops prior to getting hot enough, you adjust it to allow the handle to move further. If it's stuck and doesn't rotate at all, then, you MIGHT have a cartridge problem, but it could also be the hot line is partially clogged, and not providing enough volume to get to your desired temperature. A third possibility is if the spool valve is partially stuck. On some designs, that's a separate item, on others, it's built into the cartridge. This is the scald protection device built into any modern valve...if the pressure in say the cold line drops (someone flushes a toilet), it will sense that pressure drop, and cut back on the hot volume to try to maintain the same outlet temperature, or at worst, shut both the hot and cold off momentarily until the pressures equalize between the two lines.
The hassle with this type of valve is that it is SUPPOSED to be adjusted for a max safe temperature, but since the incoming cold may vary considerably summer/winter, you may need to adjust it seasonally.
FWIW, code says the max temperature for a shower, etc., should never be hotter than 120-degrees F...if your heat source is set to higher values, you are supposed to put a tempering valve on the outlet to drop it down. On a tankless, what might be happening is that as the volume goes up, it may not be able to heat the water enough as it passes through the heat exchanger, sort of like passing your hand through the flame of a candle...move it quickly, you may not even feel it. DO it slowly, you could burn your hand.
