the foam(?) heat shield strapped to the underside of my hood keeps sagging...

it is held in by plastic pins, as im sure you all know. my problem is the pin closest to the middle keeps falling out! i bought 4... every time i put it in, it looks fine, then i check it a few miles later and it's gone. none of the other pins have this problem.

has anyone else experienced this? also... does it matter much that the shield is resting on top of the intake manifold?