It does the same with me and really gets aggravating... but if I open the crafting menu (Tab -> Scissors [crafting] Icon), then hit F2, it works- most of the time. I might have to craft one item then hit F2 to get the "Trainer activated" sound.
To get the free trade to work, I have to go to a merchant that handles most things (not the leather guy), and with the trade menu open, then hit F1. If I'm in the correct menu with the correct merchant, I'll get the "Trainer activated" sound.
It's a hassle, but it does work.