I've been using a Yale XTC rope for two years without any problems, but recently my 4/2 Blake's hitch started slipping. I started using a 5/3 version instead, and thought I'd solved the problem, but today it started slipping too! What's up with that!?

I know the knots were tied right, I double and triple checked them (although I am still uncertain as to how tight one needs to keep a Blake's hitch). The rope certainly doesn't look worn, and it isn't glazed or anything. It doesn't seem to be terribly dirty, but there was a lot of dusty crud settling over me and the gear (this time, but not when the original 4/2 started slipping). Also, it seems to happen mainly on descent, not ascent or dangling.

What factors can cause this slippage, and what measures can be taken against them? Should I try washing the rope? Do something different with the knot? Help! Climbing is already thrilling enough; I definitely don't need the extra rush of adrenaline that this causes!

Brad