craftgrrl | More Marble Magnets / Modge Podge Question

Okay, so I saw this project idea here and I had to try them out. I know it's sort of old news to you all, but I haven't crafted much in years. This was lots of fun. :)

Question though... when I went shopping for supplies I didn't know what glue to get, so as I was pondering I saw a copy of the Michael's project idea book. Surprise, surprise, they had marble magnets in there. They recommended decoupaging the picture to the glass, but then using epoxy to attach the picture to the magnet. Well, I bought Modge Podge and used it for both. (I had never used Modge Podge before this.)

Now the marbles are done, but when I dropped one the other day the magnet split from the picture/glass part. I've dropped others since, but they haven't separated. Do you guys think that they are all likely to fall apart eventually (ie, I used the wrong glue and they are doomed), or that the one that fell apart was a fluke?