Compression Problems

Hi, I have a 2014 Land Rover Discovery 4 (Diesel). A couple of months ago, while I was driving on the highway, just going up a slope, the car suddenly turned off. The mechanics at the workshop concluded that it was an injection problem, so all the injectors were replaced. Now, with the new injectors, the mechanic discovers low compression. The compression values are as follows:

cylinder 1 - 150 PSI
cylinder 2 - 125 PSI
cylinder 3 - 115 PSI
cylinder 4 - 160 PSI
cylinder 5 - 140 PSI
cylinder 6 - 120 PSI

The mechanic put oil in the cylinders, and it does not raise the compression, which, he tells me, indicates that it is not a ring problem.

Does anyone have any idea what could be the cause of the problem?