Zr 'dullness' after a year+?

[CWilliams]

I got some recent feedback from a dr that said some of their crowns in the anterior region have become dull, like the glaze has worn off after a decent period of time (a year plus). Has anyone else experienced this? I haven't seen pictures or seen it in person so I can't describe it more than what I've been told. I also asked if the stain has worn off and the answer was no and it's also not consistent through all anterior cases. I use kuraray cerabien zr clear glaze and miyo stains. I'm assuming it's just the glaze? Any pointers or call outs would be appreciated!

 

[Car 54]

Maybe it's from the patients end, the type of toothpaste and or over brushing with an Oral-B and its rotary action?