Did it throw a code and have to be reset?
The moniotring works by pulling a vacuum on the system (via a solenoid in the engine compartment plumbed into the fuel vent system) and seeing how long it takes to achieve a required vacuum reading.
It the edge clocking of the monitoring circuit times out prior to achieving the required vacuum over a couple of key off/on cycles it will then set a code.
Problems can be many ...
Bad line, bad vapor canister system, bad/loose fuel cap and a few more are most common.
If this has been a problem (MIL code displayed) for awhile there is a 'History' in the ECM that will need to be driven off over time/cycles before the history will be reset and not show during an emissions test.
Best bet if it is a marginal system is to always have a full or nearly full fuel tank before going in for any manditory testing
This gives the least amount of 'free space' for the system to have to pull-down to achieve the required vacuum
Hope some of this helps ...