Like was said, heifers have a harder time returning to heat because of the demands of growing themselves and growing a calf. If they do not get enough nutrition, they will have a harder time breeding back
Heifers can also have more trouble calving. Troubles calving, anything from a foot back, to a hard pull, prolonged delivery, mal presentation can also delay breed back
Next and this might strike up some controversy but it can be genetic.
I have no idea what your calving season is like. For some it is defined and others all year long. Here we have a defined season of 6 weeks for the for the majority and a few straglers up to 12 weeks. We try to calve out the most in 3 weeks due to the off farm work load.
We have noticed that calves kept back, which are outside the first 45 days of calving are more prone to breed back issues or keeping in step with the rest of the herd. This usually starts with the first calf though.
Lastly, what was her nutrition like after she was weaned, and growing out. We have also found a direct corolation to feeding heavy and breeding problems later down the road. Sure they look good going out on pasture, but the extra fat can be a detriment to breeding and breeding on time. As well as it can shorten the life span on the heifer from pushing her to hard with feed, not giving the joints time to catch up with the weight gain.