so she would have been only about 45 days
Yes, at that stage it is pretty darn hard to call one bred and you are taking a risk in even trying.
You are basically just trying to determine if one of the horns is filled with fluid, there' no significant fetus to check for. If you do try felling for a fetus too aggressively, you stand a very large chance of aborting the pregnancy.
I don't know of too many vets that will call a female bred at 45-days, most wait for two-months. Your vet's accuracy at 75 days vs 45-days is a great example of why that's the case.
I'm not a vet but I personally won't even try to palpate at that stage due to concerns about causing an abortion. I use Biopryn if I want to know if a females is bred prior to 3-months.