
I think the test may well be fine, but the vets advice not right.
Having read up on the test it should be able to pinpoint ovulation within 24 hours before it happens. If test is done too late it will show that ovulation has occured but not when.
for this reason you start testing earlier than the bitch is likely to be ovulating, often day 8 is used in breeds that stand average. The test works on a colour comparison to a control.
Last year I had my bitch tested as she would not stand on her previous season being very sexually dominant. In my breed and bitches I normally find they will stand from day 14 to 17, so opted to test on day 10. first test showed that she was above the neutral control, but not yet ovulating. As it was a nearly result she had to be tested daily. Day 11, still iminent, day 12 showed that she was ovulating, or about to ovulate in next 24hours.
Now a bitch ovulates roughly over 24 to 48 hoir period, and the eggs then take another two days to mature, and then die in another two days.
Now many bitches will stand just before or at ovulation, others like mine would not stand until the last day that eggs were likely to be viable, day 16, this did not make the test wrong, it just shows how bitches vary, one will stand for a week from just before ovulation to just after eggs die, and another may only stand once eggs are ripe (in my case day 14) to when they die (day 16).
With this difficult bitch it was worth doing the test, as was able to give her previous rejected suitor a chance, and still ahve time to take her to an alternative stud. She would have none of the older but inexperienced male, and was only marginally warmer to the one she did accept grudgingly at the last minute.