In both cases, the specimens concerned are being grown in restricted space - one lot in a grow bag in a sheltered sunny spot outside (pictured left), and the other in a pot on a sunny indoor windowsill. It's not a case of the blooms not being pollinated - neither sets have produced flowering trusses in the first place.
My own personal theory about this problem runs as follows. The plants are being spoiled: too much water or, more likely, too much fertiliser. Think about it: the flowering/fruiting mechanism is at itsmost basic a way for a plant to reproduce and ensure the next generation of plants. If the parent tomatoes are so cossetted they think they're going to live forever, there's no point in fruiting, right?
The "treat 'em mean, keep 'em keen" theory is one I subscribe to out of necessity as well as a desire to get a good crop. I can't get to the allotment on a daily basis and rarely feed my tomatoes, which are planted in soil, although the earth has been enhanced with home produced compoost and green manures. I also begin stripping the plants of leaves from the bottom as they begin to curl. This is a habit I must have picked up along the way - possibly from the TV show Gardeners' World, but I don't really know why I do it - perhaps it has the effect of shocking the plant into action.
Have I got it utterly wrong - is there some other obvious reason staring me in the face? I should also say that the outdoor tomatoes came from Heritage Seed Library seeds, while the indoor tomatoes were seedlings I raised and passed on. I wondered whether there were any other reasons why an indoor plant (on a windowsill in south London, not in a greenhouse) wouldn't flower - I can obviously see why pollination might be problematic unless it was by a constantly open window that provides access to lots of flying insects.