Problems Getting Pre-lit Tree to Light

I'm beginning to hate my pre-lit tree 8). But this year we bought one of those 'guns' that you can use to 'restart' the lights. They have helped with about half of the un-lit strands. It's called a "Light Keeper Pro". Can't remember the cost, but well worth it! Found it at the big box store.

How old is the tree? Pre-lit tress should be super easy. :/

When I assemble my pre-lit tree, as soon as I get the bottom tier in the stand, I plug it in, then when I put the next tier in I plug that in until I get it into the right plug in. Some of these trees have more than one plug in on each tier, which makes no sense to me, other than to create problems for the person putting it together. Doing it this way I make sure it is ready to go without having to redo parts of the plugging in.