Just to add to this. There are many reasons a rejection takes place. Things like the chip as mentioned above, low score on the photo match, unable to read the MRZ, stop list, ineligibility for e-gates, security markings as well as manual intervention.

Also BF officers are not allowed to divulge the reasons for failure, consequently FOIA requests won't work and they should refuse to tell you when you ask them why the rejection took place.

Annoying if it keeps happening, but other than technique there isn't a lot you can do about it.