I’m not saying this is the problem, just pointing out how sometimes it’s not anything you might expect.

This was several years ago(over 15).
A customer of mine builds a hot street/strip 496 BBC to put into his customers mid-80’s Monte Carlo.
Motor gets dynoed, makes what was expected(don’t remember the numbers.....650-ish?)

Car has some sort of converter and 4.10’s, 10X28” slicks.

They go to the track on a test and tube day, and there are hardly any cars there. They can make tons of passes.

The car is all over the place in the 11’s....... not super close to the 10’s.
My customer is asking about shift points, rpm thru the lights, etc...... not getting any useful data from the owner, who is unhappy that the car isn’t running 10’s.
My customer jumps in behind the wheel...... runs 3 10.8x’s in a row.
Tell the owner how to launch, where to shift, etc......... owner back behind the wheel....... a bunch more 11.xx’s.
I don’t think he ever got a 10 out of it that day.

Sometimes........ the problem is........ the driver.