DB49,
We've looked at the MRP log, but there really aren't any clues there. None that I can discern. We've abandoned running the MDBT with scope of planning and moved to an SM36 created batch job running the plants in sequence. This seems to have helped, but not 100%.
I've gone through MD06 on long lists of materials and some will have a normal MRP list and then an occasional one will pop up where the list is blank. I've found no uniqueness to the materials with the empty MRP list.
We're on HANA and based on my research, there are any number of issues with respect to that out there. I'm guessing that SAP is going to have to look at this and come up with a solution.