Hi Renzo, what version of Designer are you deploying? I just opened all the above mentioned processes with latest Designer version 1.0.0.054 which you can get from sourceforge: http://sourceforge.net/projects/jbpm/files/designer/ and all of them open up fine in guvnor and also fully round-trip.
Most likely you are using an older Designer version. If you are using jbpm-installer you can change the version of designer in build.properties:
designer.version=1.0.0.054-jboss
designer.url=http://sourceforge.net/projects/jbpm/files/designer/designer-1.0.0.054/
then delete the designer war in $jbpm-installer-dir/lib directory and run
ant clean.demo
ant install.demo
ant start.demo
Hope this helps.