So, I'm no HVAC pro, and you really should talk to one to get a solution to this problem, especially because a lot depends on the exact configuration and purpose of your building. But here's my 2 cents.
Your building probably has various things trying to push air out of the building. Some of these are fans attached to various appliances, and your HVAC's exhaust fan. But it's also likely that the "stack effect" is in play.
To counteract these things, you need to have a way for air to reenter your building. In fact, you do -- your HVAC has an intake somewhere, and you may have other vents. The problem is that for whatever reason, the intake isn't sufficient to compensate for the exhaust. You need to fix that, either by allowing more intake or restricting the exhaust.
The first thing to look at is the dampers on your HVAC system. If they are manually operated, you could try to just open the damper on the air intake until the problem goes away. Of course, this will increase the rate of airflow through your building. If you want the flow to stay the same, you will have to simultaneously open the supply damper and close the exhaust damper. If the dampers are automatically operated you should talk to the HVAC people and see if they can be reconfigured. You could also increase the speed of the supply fan or decrease the speed of the exhaust fan.
If that doesn't help (or doesn't help enough), check whether your supply duct is leaking to the outside of the building. If you fix those leaks, you will increase the efficiency of the supply. If you can't find any leaks, you may need to put in a new supply vent to increase the supply, or, if the problem is that there is too much exhaust, try to patch whatever leak exists.