When i had this Problem sometimes with an older Frontbumper, i made following:
I take three tires on table, puting car on it. one tyre under the rearend of chasis, and even one under the left and right end of the bumper. Than i heat it with a hairdryer, near the side-end of Chasis. The bumper will lift his ends. I do it carefully, until the sideends of the Bumber ar a little higher, than i stop heating.
With this bumper i drive since a year.
At my car, only the side-ends of bumper will come down, never the middle part.
i hope, it will help you.
If not, place a rod between bodymount and End of the Bumper on the sides
Leichtmetall