Next cap is 400, and it should happen... Maybe you need to harvest something other than the zero copper beginner stuff, Daisies, Dandelions, the stuff you quested for when you became an Herbalist... Check the woods, dig up some St. Johns Wort or Nettle, that might do it.
As I was told, you can only get X10 of whatever level you are. So if your level 10, your cap is 100. It will go to 110 when your level 11 etc. Of course I am level 10 and its at 100, so dont know if this is true yet.