The BMW X3/X5 mirrors are particularly difficult mirrors to fit to. They are also extremely fragile. Do not overtighten the clamps. We understand BMW are aware of the issue.
The best solution is to purchase either Aero Platinum 6606 or Grand Aero Platinum 6613. The unique clamp design is totally universal, and will provide the strongest grip and steadiest view of any towing mirror on the market for many years to come.
Alternatively, if you already have Aero 3 or 4 mirrors, you can marginally improve the grip by purchasing F Pads 4664. If you have Aero 1 or 2 mirrors, you will need to replace your clamps with new Aero clamps 4589, or upgrade to the Platinum range.