There is likely a water temperature limit adjustment on the valve. This is so that you can't adjust the valve so hot you get scalded. It may need adjustment so you can get more hot in the mix.
For a showerhead, the feds limit the max flow to 2.5gpm. Depending on your incoming cold water temperature, you may never be able to get a warm shower...it may be hot or cold since you have not only the limits of the showerhead, but the flow sensor on the tankless that can interact so you won't have a continous adjustment if you can't get it to trigger. Also note, that if you are in an area where the winters are cold, or have a deep well, the incoming cold water can be really frigid. That will affect how hot the tankless can make the water, so you may have to readjust that limit (if it works for you) again in the winter, since you'll probably need nearly all hot because the cold will be sooo cold it will limit how hot it can get, and then is so cold you don't want much of it.
Another thing...all new shower valves typically have a pressure balance spool inside of them. If the tankless system has a flow restrictor, the spool may be stuck to one side, and not letting any hot in at all because the pressure is different between the hot and cold supply.
Good luck, and let us know how it goes.