I purchased a practically brand new Naniwa 3000/8000 combo on ebay the other day. Yesterday I lapped both sides and used it. This morning I pick it up and it's warped. The 3000 grit side seems to be warped the worst. The glue between the two sides has separated at the ends.

I lapped under running water and threw the stone into water with my Nortons, for about an hour, as I set the bevel on a razor.

What is going on with this warpage? Is this common with Naniwas? Anyway to fix this, other than lapping forever? I'm assuming the water drying out created the warpage, but it's a waterstone, that's pretty lame.
Thanks,