I already posted a question here, and I finally found the source of the problem myself. But the issue remains unresolved.
Arriving messages are stored in a queue until a matching activity is found. But if the activities are not activated in the same order as the messages arrived, the process will block.
I modelled a diagram which is available here, the corresponding BPEL package can be downloaded from here. The first process remains blocked on Receive2 whereas the second completes.
I'm testing on the BPS version 2.1.2
When multiple processes are communicating with each other, I do not always know if a message will arrive before or after another one. I think this should be possible, I see no reason why not: the messages are present in the H2 database, so a suitable query sould be able to return them.
Thanks in advance for your help.