New Lumber Gathering Ability

[Sulnoro]

Hi,

I have been trying to make an ability that allows a unit (a lumber mill for instance) to generate a certain amount of lumber every 10 seconds. It is meant to be a replacement for the basic lumber gathering abilities the worker units have.

My main problem is this:
I build a couple of lumber mills and they all generate lumber. I destroy one and it stops generating lumber, the rest keeps on making lumber. This is good. But when I build another lumber mill after I destroyed one, the new lumber mill doesn't generate any lumber.

I used this tutorial as a guide: http://www.hiveworkshop.com/forums/f279/multi-instancible-gui-spell-making-34393/

Could anyone help me with this trigger?

Thanks in advance, Sulnoro