Non-growing raspberries

I have had autumn raspberies in my garden for years. They were given to me by my aunt and have occupied several different locations in my garden, tolerating being moved aorund quite happily. However, they have been in their present position for a few years. Every year I cut them down in February and up they come again, giving lots and lots of large, delicous raspberries. Until this year. Cut down as usual, they seem healthy, with lots of green leaves, but they have only reached the height of 6 inches - and no rapsberries.

I have never watered them in the past - they seem highly tolerant of having no care whatsoever apart from the cutting down, so what's happened? Can anyone tell me how I can encourage them to do better next year?

Many many thanks - they will get some tender loving care between now and fruiting next year!