Shower sealant leaking after 3 months - will it just happen again?

Our shower, which appears to have been installed by an enthusiastic DIYer rather than a skilled professional, was leaking from the sealant joint between the side panel and the tray, so I removed all the old sealant (much of which was mouldy) and re-did it a few months ago.

It all seemed fine and stood up to extended leak testing, but it's now leaking from the same joint again.

All I can assume is that the act of standing in the tray has pushed it downwards away from the side panel enough times to break the seal.

Is there any specific flexible sealant made for this purpose, or will the only real permanent fix be to effectively tear down and re-install the shower?

The second photo is included just to show the cockeyed angle between the side panel and the tray, which is what leads me to believe this was put in by someone with more enthusiasm than skill.

Is the tray on a riser kit?

I'm not sure. This is what it looks like underneath:



And the yellow area demonstrates the uneven gap between the tray and the side panel, which I'd never fully clocked before. It's only leaking at the right-hand end, where it's obviously a much wider gap.

The gap shouldn't make much difference, apart from the fact that whoever fitted it couldn't do the job properly. As long as the the frame is actually sitting on the tray edge and sealed then it should be ok.

The riser kit will probably mean that the tray can move and flex though when in use, that movement will eventually burst the seal and I think that may be what is happening. Have someone stand and move in the shower and see if there is movement in the tray.

Maybe put some weight in the shower tray whilst re-sealing it and double seal it, as in apply one coat, allow to dry and apply another on top of that?

neutral curing mould resistant silicone