How do you keep plastic leveling blocks from slipping?

On my last trip, I was having trouble keeping my Camco leveling blocks from sliding on the blacktop as I was driving my trailer tires up onto them. (I've only used them on gravel sites in the past.)

I eventually had to resort to having my wife pull the truck forward while I put my foot onto the front edge of the blocks until the trailer tire began to come up onto the blocks.

My wife typically doesn't want any involvement in getting the TT level. Is there a trick to keeping the blocks from sliding? (The blocks also shot out from under the tire when pulling off of them when getting ready to leavethe site)