I was wondering if there is some facial recognition thing going on with mine. Mind you I've only used the eGate at LHR once (last week), but it took over a minute of spinning to send me away. The only reason I could really think of is that my beard is quite full on my passport, while last week I was sporting mostly scruff. Border force even cheerily commented that I had trimmed my beard.
The last three times I've gone through GE in US I don't even scan my passport - it just takes a picture and immediately prints out my receipt. This is with all kinds of facial hair.