If your puck stops lathering try your Thumbnail
I am not a big soap nut. I have a puck that is 90% used and have never had trouble getting lather from it. VDH or Williams probably but to be honest I am not sure. I am scraping the bottom of my soap stores which is very small. I decided to use it all up before I bought more. Not trying to complain about any brand in this thread.
I put the mug into storage a while ago and as I said as I am using it all up brought it out again. No problems. Then one day I couldn't get a good lather from it. I struggled for a week and then in frustration I pulled it from the mug and it for a lack of a better term didn't feel right. It felt like it had a different crust.
In my frustration I took my thumbnail and scraped off the outer layer and washed my hands with it. Not a thick layer obviously. Then it worked as expected!
As I stated I am using up the remnants of my "soap collection" and I think there was a reaction between soaps transferred by the brush or something similar.
I tried a search to see if this has been discussed before but couldn't find anything with the terms I thought of.:D