We have a fairly new home (built in 2003) with two identical tubs upstairs with identical Sterling mixer valves. One tub has much less hot water then the other tub. The house is plumbed with the new polypropylene pipes with individual runs from a manifold above the hot water heater in the basement. This problem is driving my wife crazy as she can't really use this tub as there isn't enough hot water for her. I don't think that turning the hot water heater up is the answer as we have plenty of hot water everywhere else in the house. Could there be a crimp or kink in one of these lines or could we have a bad mixer valve? Thanks in advance for any help!!

~Bob

Bob, I think you may have hit the proverbial nail on the head.
The trick now is to find which is the actual culprit.
You could measure the volumetric flow, but that could be misleading and difficult.
Can you swap the mixer valves without much difficulty? Or simply consider replacing the one that has to poor delivery. If the home is still under the manufacturer's warranty, call your builder and hope for the best. Just some options...

If you have a stop valve for each line coming out of the manifold check there first, they can go defective rather quickly. If you have a stop valve at the tub (behind the access plate) check it. Next open tub and shower valve and look for blockage (deteriorating washers of stop valves).

Since it goes to a manifold, swap lines and see if the problem moves.