I have come across the same problem, I don't know why.
And even more, when I create BPMN processes by http://academic.signavio.com/p/explorer, and export the process and then import it into jbpm eclipse,
then eclipse (BPMN Visual Editor) throws out some exceptions.
At last, I find that [ dataobject can be attached to sequence flows in http://academic.signavio.com/p/explorer], but it can't be attached to seq flows in BPMN Visual Editor]
After I have correct all errors (just by deleting original DataObject and creaiting them and attaching them to some tasks )
I import the process into Guvnor,
then a strange thing takes place again: web-based designer in Guvnor shows nothing in its panel for the process.
Can anyone give some advice?