This duplicates another bug already reported in our system:
Sencha Premium Member
Splitters in VBOX/HBOX layout not working anymore
After upgrading to Architect 2.1 Build 670, the splitters added to a VBOX/HBOX layout stopped working (as in you cannot resize the areas anymore)
Thanks for the report! I have opened a bug in our bug tracker.
This will be fixed in the upcoming 2.2 release, which is feature complete and currently being stabilized for release.
Sencha Architect Development Team