It is a known issue on ALG and departing FLR, where there are a number of reasons which conspire to make it not possible to load all baggage. Occasionally also on very hot days on the Embraer fleet to longer destinations. VCE should not be a problem normally, unless there was something like a lot of cruise passengers on board, since if they bought directly from the cruise company their baggage is loaded first. I think it's much more likely that the ongoing labour shortages at LHR meant they ran out of time and staff rather than not loaded a bin. Some USA airports also have labour shortages.