Is it possible for an EPROM (16C63) to check out as blank, program, and
verify correctly but not be fully erased?

I am working with the 16C63JW.  I have 6 that I rotate through my eraser as
I am programming,  occasionally it seems like things just go screwy.  Am I
not leaving them in the eraser long enough?  Please let me know your
thoughts on this.

Thanks
Steven Keller