Bathroom sub floor

Hi

I know sub floors has been covered alot, but I couldn't find my specific problem.
Any help appreciated.

I have a ground floor shower room which has been constructed the following way:
concrete slab. 50x50 treated battens screwed down @ 400 centers with 50mm kingspan in between.
In order to get the floor height to meet the entance floor height i need to add a sub floor of 28mm.
So with that i purchased several sheets of 3.5mm and 25mm wpb ply.

I have layed the first sheets of ply down (3.5 and 25mm on top) and screwed them down with floor tite screws every 100mm.

with all this i have noticed a slight bounce\spring inbetween battens when walking over it. I got my wife to walk on too (as she is a bit heavier than me) and i could see the deflection in the board.

Am i missing something here? I thought 25mm ply was supposed to be rock solid.
I have 18mm t&g chipboard on the rest of the house on the same batten construction and that seems fine.

is this a problem with regards to tiling (cracking etc), and any ideas on solutions?

cheers