There's far too many variables involved to offer a solution here. But we can suggest some measurements to make that would lead you in the right direction.
First off, I am aware of some cases (as in, more than one) of brand new 10 db antennas being bad, right out of the box. So, just because the antenna is new, doesn't mean it's working right.
Next, do a swept return loss measurement and TDR of the cable. This will show any anomalies in the cable and connectors.
Next would be to measure the actual loss of the entire cable run.
Next, find some places outside the far field of the antenna, a few miles away would be good, that's relatively free from obstructions. Calculate the predicted field strength that the system SHOULD be putting out there, and compare to what's actually measured.
Finally, getting back to the antenna, be aware that just because it looks good when it's swept, doesn't mean it's radiating in the direction you expect. It could have significant uptilt or downtilt that would produce the symptom you describe, but could look just fine when you measure return loss.
Your tech is going to be very busy.