I am unable to combine 2 items that should combine. What can I do?
For several reasons it may be that case that 2 items that are intended to be combinable will not combine.
First, make sure both items are in your main inventory and not in a backpack.
Then left click one of the items, hold down shift, and right click on top of the other item, if it didn't work repeat in the opposite order.
If that doesn't work, open the Tradeskill interface (SHIFT + t) and drop the items into it (try putting the items in both windows if necessary) to see if it is even possible to combine them. If so, click the Build button.
If the items takes an amount of skill to combine then check that you have the needed skill, sometimes you may need one point above what the game states is needed.
If the items will still not combine then we may be able to help with it through a /petition.