This is probably too late for you now, but maybe will help for the future.

Here is a link that might be useful: Invisible Zipper Step-by-Step

Oh, and this I found in regards to why they might break and what to do:
Sewing On An Invisible Zipper

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by Judith Turner
Invisible zippers are the easiest type of zips to sew on. However, I remember when I couldn't sew an invisible zip. I literally would break out in a sweat just thinking about it.

They really are very easy to sew on, however what I would like to discuss now is repairs that may be necessary for invisible zips.

There are 3 main things that can go wrong with invisible zippers.

1 The bottom section of an invisible zip can come undone. 2 The invisible zip teeth break close to the top of the zipper. 3 The invisible zip teeth break where the fabric is thick from a join in the fabric

1. You need to put your zipper foot on your machine. Turn the garment inside out, so that you have the bottom section of the zipper visible, and you can see the seam that comes up to the bottom of the zip. This seam will be undone. Make sure your zipper foot is on the left hand side. Begin sewing on the existing seam, and lock backwards and forwards, before coming up towards the zipper. You want to sew as close to the zipper as possible. Sew past the end of the section that where the zip is sew by about ½" or 1 cm. Lock backwards and forwards a few times. Make sure when you lock backwards and forwards that you go back and forward over the same seam in the same line.

2. The zip teeth will break about 3" (7 cm) below the top of the zip if there is strain on the zip when you are doing it up. The way to fix this is to put a hook and eye at the top of the zip, and you need to do the hook and eye up first BEFORE you do the zip up. There is now no pressure on the zip, and it will go up with ease.

3. The second reason is that there is a thick facing on the garment, and the zip has to go over this section. Invisible zips need to be able to slide up and down with ease. Next time you replace the zip, ask the person putting it in to make it not as invisible at the thick section on the garment. This way the zip will be able to go over this section with ease.

So there you have it. Make sure that you keep your eyes out for these things occurring while your altering your invisible zips, they can completely ruin a perfectly good alteration process.