Ken...I'm not sure how new or old your brake linings are,it sounds to me that they may have become glazed or possibly contaminated by some thing.
Do your brakes work OK otherwise?
It might be an idea to pull a drum and have a look.
It won't hurt to scuff the linings to remove any glazing,and give the drums a good scrub too.
I used to chamfer the leading and trailing edges of the linings,this stopped the shoe 'grabbing'
You can check the condition of all the other parts while you're there.
