Problem with METIS : no partition for many processors


I have a problem with METIS. I have a block structured parallel CFD solver (NSMB). I have built the graph of the block connectivity with a weight on the vertices (size of the block) and the edges (size of the data to be transferred). I use METIS to partition this graph and optimize the loadbalancing. For small graphes it works fine. Then I have a case with 660 blocks and it's not working : in the output of METIS I have some processors without any block. It works until 110 processors. Even the 660 Processors fails.
Does anynone have an idea? I can provide the graph and a small test program.