Black Moor's One Eye Not Bulgy (Looks Deflated)?

I just got a black moor goldfish and I noticed recently that one of its eyes have become non-bulgy (the eye looks normal but it doesn't have the characteristic bulgy look these goldfish usually have)... I'm not quite sure what has happened because the eye still seems to be there. I'm not really an experienced goldfish keeper so I have no idea what this could be? Can there eyes pop?

Any ideas?

It can happen that telescope eyed goldfish can puncture the sac that makes the eye stick out. Keep the water very very clean and it should heal up and refill, although maybe not as large as it was.