Not much to go on here Aina but the three possibilities that do spring to mind are 1) a touch of mosaic virus 2) too wet i.e. too much rain at some point in development and (the most likely) 3) they are about past it if the same canes have been cropping for 5 or 6 years. Clearly, if it is the weather, point 2, then there is nothing you can do but 1 and 3 have the same answer - replace them with new canes, either bought or, if you are a fairly experienced gardener, far cheaper is to use their own runners. Just in case it is mosaic (but you don't describe any other symptoms like leaf blotches, so I doubt it) then if you can use a new area then do so. Best I can do on current evidence but someone may think of something else.

Mine are doing exactly the same thing, they always have. It's due to the weather whilst the buds are growing. hot-cold, dry-wet. It's only my lates that it happens on, the earlies and mids are fine. But I tend to think that if they taste fine, what the heck. There's no sign of problems on the leaves or stems. So I just let them get on with it.