How do increase hot water flow in my shower.?

Surely a hot water on demand system has it's own pump. I would have installed the second pump after the boiler. It would seem to me that you have one pump fighting the other. You may also like to check that your mains tap is fully open. By putting the new pump before boiler it would appear that you are pulling water away from cold system. 16m is a long way for boiler to push water. You must waste a lot of water before you get the hot through.