
The ones that give exact readings are very accurate and reliable (ELISA tests) done at a laboratory.
The in-house colour change ones like 'Premate' ones are good give or take a day, but not good enough for AI for example.
Bitches will often stand for other bitches and neutered males before they will stand for a male for actual mating, and many, especially those who have had pups before will stand sooner than they are actually ready.
Also bitches can fail to ovulate or maintain sufficient hormone levels to maintain a pregnancy causing them to miss.