I would agree that it is common to have some not even show signs of heat, but out of all those it isnt very likely that NONE showed signs. But as you said, with the way they calved it absolutely sounds like thats exactly what happened. I would call it a fluke and expect some heats this year! With them being second calf "heifers" now, i would say a 60 day post parturition should be adequate to re-breed. Given that body condition is good. As it was mentioned, if these cattle for whatever reason arent showing heat this year, I would try a TAI. Using CIDR's of course.
And now that I think of it.. I kinda had the same thing happen several years back. Not many signs of heat, only bred the higher end cows TAI to save on semen if it didnt work, most of those calved with the AI dates and the rest calved together with the bull.
Anyways, good luck!