Recently I noticed that the water in my master bathroom shower does not get hot... barely even warm enough to take a shower. Everywhere else in the house, including the bathroom sink, the water gets hot right away. There are no problems with water pressure, just that the shower water doesn't heat up like it used to. I bought this house within the past year and it was a total rehab, so everything, including all the plumbing and fixtures are pretty new. It has a tankless water heater on the floor below.
The shower itself has separate handles for turning the water on/off, and then another one to adjust the temp. If I turn the temp towards cold it gets ice cold, and as I turn towards hot it gets a bit lukewarm but that's it, even when I move it all the way to hot. It wasn't like this before, just seemed to happen recently. Any ideas would be helpful

What's the brand name of your shower valve? How old? A model number would be nice.
Regards, Tom

I believe it's a Kohler HiFlow Rite-Temp... not sure the exact model number. It's a one-handle control of temp. The house was rehabbed during 2007-2008