I think there is nothing wrong with the buckle the part of the buckle that looks transparent is to deep in to the shoe, to be sure you will have to bring that part a little bit more to the front, the way it is now it partly takes  the color of the shoe, which gives the impression it is transparent.

Hope I explained it well, but that is what I think happens, if you putt the buckle a little bit to deep it looses his color and takes over the color of the part beneath of behind it.

big hug and I hope it works