shower tray

Have recently fitted a foam backed shower tray which i found very flexible when i fitted it , its a 1200 x 900 tray and 40mm thick i stuck it down with 10 tubes of evo stick sticks like s**t onto 18mm marine ply supported by 4"x2" batterns , the shower has been in use for a week i was called back to seal up a small leak on the shower when i felt a high point on the tray i can actually push it down with my foot ,it is not causing any drainage problems and is away from the standining area ,has anyone any ideas why this area has risen even if there wasnt any adhesive under it surely it wouldent have done this.