oh man i don't know if this is the problem for not working but this event: (Ability being cast) Equal to Combine, should not be there. You should place it in the actions-condition.
I'm sure it should work like this also, but it may create problems when having more items that you would want to combine, since you already do the actions, before checking for item count.
In fact I would do the triggers very differently, just check for items, say units acquires item of type stick, do all the actions you are doing now, just in the actions-condition place unit is issued an order to cast an ability of type combine, as well as the current condition you have.
But yeah, if you stick with this trigger, make sure you have a integer variable.
Also did you place the Jass code?