Well, the way I see it , you have 3 options.
1st, I would talk to the contractor because that is technically his responsibility. 2nd You could try prying out the edges of the surround and squeezing some glue underneath. Use a high grade quick set glue.
Once you get the glue in place, you will need to apply pressure with a wedge board.
3rd Remove surround very carefully and re install.
Hope this was helpful