Disconnect the hose from the tray - if you are getting good flow of the foam - I'd say you probably did not lower the shampooer all the way down.
If the foam is not being dispensed continuously - you may want to check:
1. Check the suds screen
- make sure it is clean and installed in the tank and is clean f lint
2. Check the sponge where exhaust is inserted in the tank - rinse it. Also look for the clog under the sponge in the valve.