I have some 4 year old Glenn Ample. This year the fruits were not very big, although there was plenty of them. The fruits last year were a good size.
Should the rsapberry canes be replaced after 3 years ? I have fed them and so am a bit suprised that they are so small !
Raspberries should last 10 years at least. Some varieties are small. Mine are a big variety and this year are huge. I put that down to watering, with a soaker hose around the base of the canes.
Many things which would have been plumping up when we had the long dry spell seem to have suffered this year, unless they were watered as fidgetbones did. This year's fruits on my recently-discovered-and-rescued-from-brambles tayberry were small and hard. So I have just cut the fruiting canes right back to the ground and spread out the new ones. Hopefully next year they will do better.
Chris, the roots could have dried out a bit at a critical time during the hot weather. Raspberries are very shallow rooted so can be affected this way. If you mulch the ground with a few inches of compost early in the year this can help both feed them and keep the roots from drying out.