Photinea wants to get HUUUUGE Jadders, and I suspect it has got to the end of the line. You can bonzai almost anything you want to try to keep very small. Put a vigorous shrub in a pot and it will not live to it's full life span, unless decanted, resoiled, root pruned, fed and maybe placed in a larger pot, watered and cossetted.

This dwarf form of Photinia (assuming it really is 'Little' red robin and not Red Robin) should only reach about a metre tall - I didn't think, as a cultivar, it had been available for ten years, thought it was a fairly recent introduction, but maybe I'm wrong. It's not entirely hardy either, so may well have suffered during this last winter, but it sounds as if its outgrown its pot - I'd cut back hard, turn it out of its pot, scrape off all the old compost, and probably a bit of root at the same time (not too much) and then repot into fresh compost and keep watered and fed to see if it responds. On the other hand, if its actually Red Robin and not the Little variety, the only option is to replace it or plant it in the garden, having cut it back.