Question I loose enemy bullet interactions but no idea what causes this

Hi all,
I have a mobile (android) tower defense game.
Everything works fine.
But something is happening and enemies become indestructible.
Suddenly, even though bullets hit them their hp is not reducing.
Here is some more information.
I use 0.04 fixed time step (for performance sake)
I use ondemandrendering=3; and application.framerate=150 combination for performance sake.
and my default solver iteration is 1 (instead of 6) for performance sake.

Obviously, I am killing physics performance by changing these values. But it works great performance-wise and normally there is no problem.

But what can be the reason for changing bullet behavior and stopping hp reduction?