Can you be a bit less cryptic?
How about some basic system information, such as:
The make/model of boiler?
The make/model of zone controller?
Separate pumps for each zone, or single pump + zone-valves ?
Amount & type of radiation for each zone (separately, not just one number)?
By "...unless i turn the thermostat for the main zone off..." do you really mean "off", or do you mean "turn down so that it isn't calling for heat" ?
I could speculate all day on what could cause it, but it's crazy to run through the possibilities without some real information.
Here's just one possible explanation:
If the zone controller one that has a "priority" zone, and the main zone's thermostat is on the zone designated as priority, all other zone calls will be ignored until the main zone's call for heat is satisfied.
There are others.