
If the mating lasted 6 minutes without them being held together then I would not call that a slip mating, just a brief tie.
It only needs to be about a minute to be successful, which is why sometimes slip matings produce results, but sometimes not.
I would agre, unless this is a bred where the bitches are big and the boys small and ties are less common, then I would say she was not quite ready and you should hopefully get a good tie today or tomorrow.
As a bitch owner I would certainly not be happy with just a slip mating.