Substrate thickness problem in patch antenna

I'm designing microstrip patch antenna in cst mws.
At the beginning i used microstrip transmission line to feed the antenna. I needed to get good results and so i started to increase thickness of the substrate. But not only bandwidth got better, radiation efficiency did too. But i was supposed to get decreasing in efficiency due to theory.
Then i used coax probe feed. And again i got the same results - increasing subs thickness gave higher efficiency.
Please, anyone, help me to explain this problem.
i use fr4 subs, freq - 2 ghz.

I suggest you use GE FR4 called GETEK which has 10x lower loss tangent than most FR4. W
I suspect you are using lossy FR4 and also dielectric constants is much lower at 2Ghz since rated parameter . Also dielectric constant drops 10~20% as f goes up.
Return loss of thinner patch may be worse if variations in thickness are more, and variations in dielectric thickness are more.
PLs post test results for return loss? resonant frequency? bandwidth?