Rear bumper needs adjustment

Hi, the bottom edge of tailgate on my 2014 HSE is very close to the bumper when closed, so close that it can almost touch. The paint has rubbed off and started to corrode. I have it booked in next week to be treated and painted.
To prevent it from doing the same, is it possible to adjust the bumper down by maybe 5mm. There’s no sign of accident damage that has pushed the bumper up.

Many thanks for any suggestions
Ian

Check for dirt under the protection trim. It's a real dirt trap, which in time pushes the bumper up towards the tailgate bottom.

Thanks 'Nodge68' for your suggestion, I have already removed the trim and it was quite clean underneath
Ian