fitting Topeak Defender front mudguard

skyeflyguy

HI,in the process of fitting some Topeak defender R1&2 mudguards to my winter bike. The rear guard went on fine, but there is a little plastic compression fitting that fits into the rear of the fork in the recess where the brake bolt goes through. No matter what I do the fitting keeps popping out when I'm adjusting everything else.
Has anyone else encountered this problem, and any advice appreciated.

Thanks.