why doesn't any steam come out of my simac steam cleaner?

I have a simac vaporsimac PV20 steam cleaner, which has been used at the most four or five times for carpet cleaning etc. I got it out before Easter to use it and though the machine heated up no steam came out when the pistol grip was released. I tried three times with no luck and had to release the steam built up manually, which was a hazardous procedure.
Lakeland ,where I purchased it were very helpful, but suggested I ring the helpline first in case there was a simple solution. Alas, there was no one there and will not be until the end of April. Can anyone help me? I am really fed up with this as it is the second steam cleaner from this company which has failed.