The question has several possible answers. Most of them are obvious to anyone familiar with PROFIBUS networks, so we won't mention them all here.
However, there is one remote possibility that has been known to cause problems in the past that might bear checking. The parameter that should be checked is found in the Master Properties window, on the "Bus parameter" tab and is called "HSA". This parameter sets the "Highest Station Address" with which the PDPMV1 will try to communicate.
If this value is set too low, below the Node Addresses of some of the nodes on the network, then those nodes will not be accessable by the PDPMV1 Master. So the "HSA" parameter needs to be set to the highest node number that exists on the network that the PDPMV1 needs to access.