Re-sealing won't help if this is a standard plastic tray and the problem is that its coming away from the tiles - it'll just fail again.
It seems clear that the repair hasn't worked, so getting the builders back in seems pointless, if cheap.
Some personals.
Showers are made of plastic and plastic can deform under load - so, are you very heavy?
Sometimes shower baths need timber carcassing to support their base on the "inner" sides, or else these can deform - was this necessary or was it done if it was?
Some advice.
This problem has gone on for a while now.
If untreated softwood substrate was used, this could be affected by the leaks.
I'd ask the builder to open up and prepare a record of who attended on what dates.
It could be that it was the plumber or his mate, not the builder and no records may have been kept.
If you can show this problem was not sorted from the outset and has continuously recurred, you may have some comeback.
However its more than a year since you moved in and the defects liability period is usually one year, so you may be out of luck.
FWIW
ONQ.
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