It's a "long" bug i guess, i reported it long time ago and it seems it's not solved yet
mm75
The source of the bug is that the weights of the block that updates Will's Flail is set to a value too high, so that block of code never get a chance to run. Easiest way to fix is: open the ~\NTotSC\D\KELLDA.d file with Wordpad, locate the block of code "BEGIN Tells", change "IF WEIGHT #4" to "IF WEIGHT #-1"(Actully it is the second line of code in the file, right under the line "APPEND ~KELDDA~"). Then recompile the KELLDDA.D file then it's done. Remember you can only upgrade Will's flail after Chapter 4(not in chapter 4, but chapter 5, 6, ...) and after you have finished the Bassilus and Wyvern quest.
To recompile the file, first copy the latest WeiDU.exe in your BGII directory, open Command Prompt mode, then go to your BGII directory, type
WeiDU NTotSC\D\KELDDA.d NTotSC\Language\english\D\KELDDA.TRA --out override
then Enter. This will create the correct KELDDA.dlg file in your override directory. If anything goes wrong, just delete this and you will be OK.