I have the same problem and only ever at LHR. The last time it happened I was told it was because I have my passport in a thin plastic cover, but even without the cover it doesn’t scan. One time coming through Passport Control I was instructed to use the eGates, explained to the lady that my passport never scans, but she made me do it anyway. You guessed it, didn’t scan and I ended up at the back of the queue to see a human.