QUestion about Williams TrueBlast Sound

I recently purchased two new Williams diesels, an F-3 AA and an Alco FA AA. When I first ran them and tried the horn it worked fine with the usual TrueBlast sound. However, after a couple of horn blasts on each set, the horn stopped working correctly and now only bleats once sort of pathetically.

I am using a Lionel postwar TW transformer which activates all of my Lionel horns and whistles from both postwar and modern productiuon just fine.

Any suggestions from you gents? Thanks!