Theory and prctice mismatch

Please, someone, need help with next problem.
I'm designing a microstrip patch antenna, freq - 2 ghz, subs - fr4 with cst mw studio.
When i increase the height of the substrate(from 0.4 to 0.8 and to 1.6 mm) radiation efficiency increases. But in theory it must come down with increasing the height. What is the solution of this?

There are a lot of factors involved in it, you cant say that just by increasing substrate thickness you will get an increase in radiation efficiency. If you increase the substrate thickness then the theory of quasi TEM is no more valid and things like this. It is complex expression please review it again.

But could you give me some ideas why it increases?

Please read the first two chapters of "R. Ludwig & P. Bretchko, RF Circuit Design: Theory and Applications".