My house contains 3 showers. Two of them warm up to where it's comfortably warm and one just warms up to a point where I don't freeze. Even if the shower is on the highest setting it still is not warm enough. What could be the problem?
It is likely a defective shower valve, allowing too much cold water to mix with the hot. If the showers are all the same brand and model you could make a swap and verify this suspicion.
It could also be a pipe issue. If there's a nearby sink which might share plumbing, run the hot for 1 minute into a bucket, run the cold for one minute into a bucket, and compare the ratio. Now do the same at a "good" shower. This might show the same pattern of differential flow.
Check that any valves in the line are fully open.
While it may be the shower valve, it could also be a restriction in the pipe itself - if not a partially closed valve, it's a lot harder to find. I had a mystery "hot water to tub" problem that eventually turned out (as I worked my way back from the tub disassembling pipes) to be a copper elbow almost, but not quite, completely plugged with solder.