What's Wrong with Quenty's Welding Script?

I've been scripting for years, and I've never encountered such a weird problem.

So I was using the qPerfectionWeld Script (which I've never had a problem with) when I discovered that one of the parts in my tank wasn't welding properly. Here's a gif: (notice how a Part at the front isn't moving).

Why is this? What can I do to fix this? I've taken a look inside qPerfectionWeld but everything's fine.

If it helps, I took a look inside the Explorer and found this. I tried clicking on it, but to no avail.

It would be greatly appreciated if somebody could help me. I haven't edited the qPerfectionWeld Script at all.