In a recent change, it was set that herbs could not train your stats above their potential maximum. In the current implementation, however, there is a workaround that allows herbs to continue to do this. It works like this:

1. Raise your stat to one point below maximum 12/13 for example.
2. Use a large amount (>20 is usually best) of the appropriate herb.
3. This will generally give multiple increases--the first increase will raise your stat to 13/13, the second increase will raise your potential to 13/14.
4. Repeat steps 1-3 until your stat reaches the hard cap at 25.

This method effectively gets around the potential problem with herbs entirely. It is possible for the method to fail (if you don't get a second increase), but this can still be circumvented--you can simply get your stat drained (either by an undead for To or by overburdening for Dx) and then try again.