I have an installation of hardwood in a new home, that has developed some deficiencies.
Most are site related, as determined by our service department, and agreed to by the builder.
One issue that we are not completely sure of are several areas that have signicant end gapping. An independent consultant hired by the home-owner states that this is "installer error" but we (I, mostly) disagree.
The material is pre-finished oak, house was installed in the fall, approximately 3 months after the foundation was poured.

If the flooring was solid, it was installer error. If it was engineered, it could be faulty material or improper acclimation or even failure by the homeowner to maintain proper humidity levels or installer error.

Thanks for your input Gary, but perhaps I wasnt clear enough in my description. The end gapping was not evident during the PDI at the time of closing. They have appeared since the client occupied the house.

Hard to say without being there and doing on-site testing.

The wood may have not been dry enough when it was milled. That is my first thoght because you don't mention any gapping.

Was the house conditioned to a lived in environment before the wood was brought in?

Was the wood brought in early to acclimate?

Did anyone document any moisture readings on the subfloor and flooring?

Are the homeowners maintaining the RH levels adequately?

A certified inspector should have a background in wood science to help determine the underlying cause.