Ensure you have water in tank and solution (or water in solution tank). When you start the vacuum up, the pump starts to prime itself. If you have air in the system, it takes a little longer to prime. Allow the vacuum to run for 1 minute before attempting to pull the trigger. Then pull the trigger.
If this doesn't work, you'll need to open the unit up and check the hoses for any clogs, and next test the pump to see if it is functioning.