there's different kinds of nylon straps, different types of buckles. Cheap straps made with coarsely woven, loose strapping tend to slip.

I like fastex bucklelocks myself, the kind backpacking sternum straps use, but i try to pack light enough i never need to strap something big like a sleeping bag or tent to the rack, just drying clothes and stuff. which usually stay put with a flat bike bungee.