In reply to Martin1978:
I'ved used it with 8.5mm 1/2 ropes regularly and never had any slippage.
I'd check for wear, and also think about the diameter of the carabiner being used to create the friction - could be it needs to be wider diameter, or a rounder shape rather than I/T bar type maybe.
Also make sure nothing could have been levering the device (ground, cliff face etc) and partially releasing the grip that way.
What kind of nick are the ropes in? If they're old and shiny, or new and slick that could be the problem as 8.5mm is right at the low end of the diameters the R3 is rated for.
Cheers,
Stuart