Were you applying alot pressure to the hones when you were lapping? If so, that could have helped shear the diamonds from the DMTs edges. You pretty much only want to use the hones weight when lapping, though if a stone seems to need alot of work I will put the DMT on top, since its pretty heavy.
I agree, use no pressure when lapping, just the weight of the hone. I've never had a problem with my dmt.