How are you heat detecting? Once a day? Twice a day? Are you using tail chalk or a patch? Give us more detail.
We AI everything, and use natural heats. We had one heifer, a May, that we never saw in heat but would see her with a little sting of blood every 21 days. We could not use tail chalk because she is my daughter's show heifer. We ended up putting a CIDR in her after Nationals (mid-July - used the 7 day protocol). We pulled the CIDR on a Tuesday at 2:30PM, she was jumping other heifers on Wednesday evening, and standing good by morning. We bred her that night (to Built Right ;D), and she passed over her 21 day cycle last week. We are now done showing, so I put some chalk on her tail to keep a close eye on her since she has a history of not being seen in heat.
My point is, some are hard to catch, so use every resource you can to watch and catch! Checking three times a day really helped us narrow down when to AI, since we caught them coming in at a little more specific time frame..