You're not alone, my partner's passport has had zero success getting back in this autumn, mine has however not had a problem, both mid 2016 issues. Staffing at the gates leaves a lot to be desired late evening as well, especially when all the based aircraft arrive back in one long stream.
Mention it to the the Uk border staff and you get shrugged shoulders and "it's the airport's problem", mention it to airport staff and the shrug is answered with "it's a Uk border agency problem" Ho hum, welcome back to Bristol...