I have Stack Size Controller installed. My stack size of large-furnace is set to 10. I have added large-furnace to vending machine controlled by this plugin but only 1 piece, not 10 (not full stack). In this case when someone buying large-furnace, he got it doubled in his inventory and in vending machine I got nothing. Seems like plugin is "restocking item in wrong place" :)
If I put 10 pieces of large-furnace, then everything works like it should. large-furnace is just an example. So to get it work correctly items in vending machine should be full stack, to avoid this bug. Maybe you should add this to description, or fix this.
Thanks.

i found this to be a problem too, its an easy fix, one line
line 80 change

Then you only need to add 1 item in the stock, and it seems to work (the false is for stacking)