I know of one store that was using an aragonite plug from one of the e-bay sellers that had the same problem. They switched to the ceramic plugs and the problem went away. I see a lot more stores using these over the concrete based plugs.
http://www.thefilterguys.biz/frag_supplies.htm
If something is cheaper it isn't always better. You just never know what is mixed in with the concrete that is used to make the plugs. I know of guys that have been using the ceramic ones for years with no issues.