Why does the sealant keep failing at the shower tray?

Because the cause is under the sealant, not in it. Three things account for nearly all of it: the tray moves (bedded on blobs or unlevelled legs, so every shower stretches the bead), the bead was applied badly (onto dust, unloaded, or too thin), or there is no upstand or seal strip behind the bottom course of tiles. A second bead over the first does not bond and traps the water already there. Grade it medium if dry, high if there is any staining below.