Does the wedge appear to be deforming in shape at all? Maybe one side starting to bend or any other abnormality?
I had that happen on a Pietta 1860 and I don't know what caused it. I suspect it was probably user error of some kind. Anyway, I ordered a new wedge from VTI and the problem never happened again.
I shoot 20 grains pyrodex in my 1851 routinely ( about 400 rounds so far) and have never had a problem with the wedge.