So breeding 1/2 brothers to 1/2 sisters makes offspring infertile?14 pg of 18 exposed = 77.8% pg rate
17 expected pg of 18 = 94.4% expected
IF bought at your local sale barn, I expect the owner sorted off his best most fertile heifers for himself. In addition perhaps 1 or 2 born twin to a bull might have been put in the group. When working with a small number of animals just a couple of them can really skew the percentages. I agree with LazyM's suggestion of bagged mineral. Otherwise, don't think it's your fault. Unless the 4 were smaller I doubt they were too young. More background information when buying/selecting is always helpful.
Knowing the seller or whether they are prone to lax management is a red flag, leading to another possibility - inbreeding lowers fertility.
IF heifers in the group resulted from 1/2 brother x sister mating or sire x daughter that could well be the reason for less than a 90% group pregnancy rate.