Well in all honesty that was just one item. I've been having trouble +ing on several items, and with me more people. But as I said in the global I will try +5ing with just prem+ see if that works better. To be able to absolutely say 'nothing is wrong with the alchemy' you should make atleast a dozen of items to +5 though. So you have a decent control group.
Also if you don't mind me giving a little tip, holding alt while left clicking items will quickly move them from one inventory to the other. So just hold alt (I use left alt) then click the item, elixer and powder in that order and they will all be move to the alchemy screen. Then you can just hit the button. Next + all you have to do is leftclick the elix and powder while holding alt and voila. It can speed up the process quite a bit.
Also if you don't mind me giving a little tip, holding alt while left clicking items will quickly move them from one inventory to the other. So just hold alt (I use left alt) then click the item, elixer and powder in that order and they will all be move to the alchemy screen. Then you can just hit the button. Next + all you have to do is leftclick the elix and powder while holding alt and voila. It can speed up the process quite a bit.