Photo Charms using PMC

Hello, everyone -

I’m new and have a question. I’m making a few photo charms using
precious metal clay. I’ve been sealing the photos in the charms
with Envirotex Lite.

Most of the time this works great. However, on a couple of charms
the Envirotex Lite has turned yellow and has started to peel away.
Does anyone know what causes this and how I might prevent it? Is
there a different sealer I should use instead? I thank you in
advance for any advice you can offer.