New MX Durastone shower tray, near waste making a creaking noise

Hi my floor is concrete so built a timber frame (timber 60mm x 70mm) with about 100mm between each section secured to concrete base. Also made opening in first two sections for waste unit and for access. Checked all level then screwed down 18mm plywood base on top across all sections of timber, confirmed all level again and could support my weight (!!!).

This I need help with. Instructions say use an adhesive, so used uni-bond mega grip. Used two lots with wide hole on an up and down movement across all of board. Layed shower tray on top and weighted down by standing on it. Left for a day checked no leacks again and fiited peace of timber to fill and support access space. Problem is when we stand near waste (only waste) the tray makes a creaking noise. My thought is that glue has not stuck there and that's the problem. My question is could this cause a problem to the tray i.e crack, as to fix can only think of somehow taking the whole tray up and trying again to stick down or just put up with the noise.

 

Thanks for reply, but had a thought. As I'm able to get to waste unit and remove if need be. Thinking in doing this and trying to pack with plastic shims between ply board and timber supports around waste area to see if helps. But yea I'm not bothered its my other half, lol

 

*** UPDATE PROBLEM FIXED ***
Like a lot of questions where answers are given or suggestions you don't always see a follow up, but here is mine.

Firstly I have learnt that with these tray's although light and flexible they are also a lot thinner around the waste.
If doing again I would put a larger bead of adhesive all over the tray with a bigger bead around the waste, also would put weights on the tray mainly around the waste area. Maybe using bags of sand on a blanket, until stuck good over night.

Anyway getting back to my fix, as I was unable to take tray up (most of tray stuck good), I found that I good wedge a metal ruler between the plywood and bottom of tray from going through from the front of the waste access. So I knew that the gap was between 1 mm and 3 mm. I bought a pack of assortment plastic shims from screwfix. I managed to push a couple of 1 mm and 3 mm shims into the gaps (using the metal ruler) and to my great excitement this has fixed the problem.

So hope this helps others who may experience the same problem.....