Just for background - I've forgotten the details but from memory Long haul turnrounds are typically set at a minimum of about 1 hr 30-45 min (more if an A 380) so anything inbound more than maybe 15 min late and the outbound will almost certainly be delayed.
As to why the Boston-London is consistently delayed - you need to look at the aircraft's flight history (and flight radar can do this) prior to the London-Boston sector to perhaps get a handle on the problem, reason being one possibility of several is that the aircraft is being planned for a whole series of tight turn arounds and the delays on the Boston service are the end result....
One problem almost unique to Long haul is that the aircraft don't get a night off that can be used help reset schedules, so that in effect they can start the day afresh and ensure the first departure is on time.
The reality is it's not at all uncommon for a Long Haul aircraft to arrive at LHR from somewhere in the East at 8 in the morning and be scheduled for a US departure two hours later....so a big delay half way around the world can end effecting an arrival time into the States best part of a day later.
Sometimes it takes a complete rejig of the integration of an aircraft's schedule to fix a consistently delayed service, and given the tightness of resources that often can't be done quickly.
Not sure if that helps because ultimately it's PITA I know, but hope it's useful.