Make sure your Icon Bar command shows: use_itemvdl 3217595963
I'm getting good casts on the Icon Bar, Gear Buff Macros and hotbars, before and after entering an instance. If you add Brew of Readiness to the worn item ID on the Gear Buff Macros window, it will give you the error message "Item Brew of Readiness cannot be equipped in that location." Is that the message you were seeing?
Also, are you getting a buff on you that resets cooldowns? I know there are spells that do this, and they may be on the same timer as the brew.