I'm no expert, but I think I've read something along these lines...

I believe some raspberries produce a fall crop AND an early summer crop. The fall crop is produced in year one while the summer crop takes 2 years. It is likely that you are experiencing a small fall crop that isn't reaching full maturity, but hopefully you will get a bigger crop next summer instead. In essence, your fall crop will never reach maturity but your summer crops will. It's just that they're delayed an extra year.

Hope I'm right. Next year you'll know for sure.

Is the patch on a slope, did the fruiting plants have the best soil drainage? Did all of the plants bloom equally?

I have a similar problem, my guess is too much moisture. My plants do not get morning sun, and stay wet from the dew longer than they should. They seem to have a disease, and are slowly dying out. A few years ago I had a drought, and that was my best year. I have new plants ordered for a new sunnier location. Your plants on the end of the plot produce more, maybe they dry sooner than the others from the sun, or, as stated above, they might have better drainage.

Benny

A raspberry patch that slowly dies out over a 2 or 3-year period most likely has wet root problems.

Even in a small, flat garden area, you will notice some small puddles forming in the same spot in winter year after year.