Two reasons, mostly:
1. Template Monster templates often edit core files in addition to overridable template files, so there isn't any way for us here in the forums to test changes before we advise you, as we can with changes made in the stylesheet. We would have to have the template in question installed to offer advice, and it obviously isn't reasonable to expect the volunteers here to buy all of TM's templates.
2. You paid good money for that template. You should, it seems to me, expect the people to sold it to you to help you fix it if it doesn't work. Too often, they won't, which is another reason most here recommend against using them.
All that said, many people here, including me, routinely help with TM template problems.



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