Probably a foreign body blocking the valve open. or stopping the seal,
easy to find out just undo the nut on the bottom of the valve.
I don't know how old your Wonderwand is, but the spring can weaken.
Mine did at about 18 months old. I packed a washer in the nut to pre-tension the spring.
Worked great for 6 months, just changed the valve now, although it was still ok.
For the price you may as well just replace the valve.
http://www.wcsuk.com/acatalog/Valves___Repair_Kits.html