Older raspberry plantings can sometimes experience a drop in productivity due to a variety of reasons. It would help to understand more about these raspberries and your management practices. How long have they been in the ground at this site? How and when do you perform annual pruning? When do you fertilize them and what is the analysis (N-P-K) for the fertilizer? 

I appreciate the response.   Brace yourself, your answer may be to simply send me the paper on how to grow raspberries.   I had no idea that they needed to be managed

I took some from Saline MI and planted them in Cass City MI in 2001.  No pruning, no fertilizer, no sail tests, no thinning.  they are in full sun, loam soil, not in rows and take up an area 20-30'

Thanks for your help.

They are just for enjoyment - not commercial.

Thank you

Ok, no problem. I have attached a book that you might find helpful. We also have an online class called backyard fruit 101 that has a section about brambles. You can register for that class here: https://www.canr.msu.edu/courses/backyard-fruit-101
If you enjoy the no (or low) management approach, one simple thing to do this winter, would be to just go through with sturdy clothes and leather gloves, and prune out all of the old dead canes from the patch. Any cane that is older than 1 year can be removed. You should be able to identify the 1 year old canes by their color in the winter. The attached book will have illustrations and photos that will help too.