License and glazing issues!

Ok... just a quick post and update since it is just after 1am and I am so tired!
So my daughters license came in the mail today... I think she stared at it with a smile on her face for like 5 minutes... yikes.
I let her take my car for the first time a couple of days ago to run across town to apply for her first job... as she drove away... I cried like a baby... as a matter of fact so did her dad. I cannot believe my babies are growing up... its so scary thats for sure.
So one other quick note then I am off to my little dream world...
I have been experimenting with creating tile pendants.... to sell and to create with and for my two teenage daughters (they love the look of them so far)
I used both scrabble tiles and glass tiles (4 of each) to see if I had a preference and to see how they each look. I chose my base, chose my art, mod podged my art to seal, then mod podged my art to the tile... set and then used diamond glaze to create my glass like domed finish... only when the diamond glaze sets... I end up with this center concave on my tile... so irritated!
So I asked around, google searched and posted in forums... no one with an answer and google turned up no real results for my issue. I tried different techniques on these tiles, different thickness of the lay of glaze... still each concaves. So I added another coat... still the same issue happened... so another coat... still the same results... ugh... one last coat focusing on the concaved area and spreading so lightly with a paint brush to the edges.... this should do it for sure is what I thought... 5 hours later... blah... same thing... CONCAVED!!!
So my bf is trying one... so far... no concave issues... been about an hour and a half... what the heck...what did I do so wrong with this... but I am going to wait until morning to see for sure if his will do it or not (concave)
We may have to move on to a different glaze... if that doesn't work.... RESIN... Yuck!

Well any suggestions are greatly appreciated...
so much for short and sweet!

Good night!