I wrote the bug report on that after d3 2.0 came out:
Make sure its added to the potion slot on your d3 action task bar
Make sure you have atleast 1 normal potion in your inventory. (I dont know if that has been fixed yet, but originially you had to had 1 normal potion in your inventory for the bottomless potion to be used.)
If it still doesnt work please post under trinity thread plz, Trinity handles that.
Thanks