@earthowned & @davecykl Thanks both, but I've tried the whole rotating the wheel thing. I've managed to lock bikes at virtual docks lots of time (the nearest dock to home is one, so I get plenty of practice), but sometimes it just doesn't want to lock.
Agree the tabs without the plastic cover don't help.
@Snowy "When I tap 'end trip here' it usually just says 'communicating with bike' (or words to that effect) and then asks me to lock the bike - that's the point the lever can sometimes fail to budge. " <-Yep, it's exactly this situation. IIRC, the bike lights are flashing in a way that indicates it's ready to lock, but the lever doesn't budge, even after spinning the front wheel a few times.
One of the bike team fellas told me that they sometimes lock better when the front wheel is pointing straight ahead. Not sure why that would be the case.