
As mentioned before all of mine, bar one, went to a light straw colour by around day 10 and then, mated or not, would just have less and less discharge through the final week, usually not noticeably blood coloured as I remember. The one I had mated on days 11 and 13, and after coming home, was quite alarmed when she started producing a much more coloured discharge than I'd seen before. She was otherwise fine, and quite normal but at the time, I thought it was more likely than not, that she'd not taken. Turned out I was wrong to worry as she duly went out of season by around day 21, had an uneventful pregnancy and then, despite needing a C Section (secondary inertia) produced 9 of what was probably my best litter - the one we kept 3 from!!
Again books of mating, pregnancy, whelping etc. are just text books and bitches don't read text books. So each pregnancy, even from the same bitch, really should be taken in it's own right. And if you are worried, see a vet. I don't know the 'whys' - sorry other than we are dealing with living beings and no two are likely to be the same.
Is whether slip ties are successful or not, down to breed? I can only say, again, one of my lads never achieved a full tie, but in the small number of bitches he had, only missed to one.