This can eliminate some possibilities:
Input pressure: 15psi-150psi
Output Flow: .6gpm - 2.7gpm
The impeller will spin but no heat will be produced outside these ranges.
The gpm rate is determined by temperature rise. Measure ground water temperature as it comes into the house; let it run for a minute before you measure.
45 F degree rise will give you 2.6gpm ( 70 heated to 115 F degrees = 2.6gpm)
50 = 2.3
60 = 2
70 = 1.7
You can't control the incoming temperature but you can control flow at the output valve or faucet.
If none of this helps, then I would think you have a mechanical problem.