+1 on the above advice, good stuff, sometimes if it's bad cupping can be cured by pressing the strop between to flat boards for a day or 2 if needed.
Older strops have problems all their own. I have an older "New England" strop right now that is showing signs of haveing a "Nap", what I mean is, when stropping thier is a nap of some kind that the razor push's one way then back another way, and the razor needs wiped down as the blade has some "Stuff" that comes off the strop.
My assumption is some one put WAY to much strop dressing on it and soaked it good, Now I have to rub if down with alcohol let it try, 'not to much alcohol" and sand it down with may be 3 to 4 grits of sand paper and see were I am at that point.
Yes, I love older strops as well as some of the newer ones, but they do come with some Bagage!!
[just like people]

Me again, tinkersd