Another potential problem is the zone circulation pumps... If they are spinning and you are not getting heat then whacking the zone valves ought to help. If they are not spinning then I would be checking the connectors...
One other odd problem I have come across.. On the smaller hydrohot units they will generally only have 2 zones( two outputs) but will have three zone controls as in one in the basement and two in the house itself... I have seen a couple instances where the basement zone and one of the house zones controls are in series.. Meaning both have to be on for the circulation pump feeding those two zones to operate.
JohnnyT