The Steam Mechanism Won't Operate

Hi There,
Of recently I have come across an unusual problem where by Expresso Machine won't produce steam.  When I turn the knob to the right the Expresso Function will work but soon after I turn the knob to the left for the Steam Function the Machine will not operate where by it doesn't make that thumping noise before the Machine will produce steam through the valve.
Any ideas?  I had only de-limed it in the last month or so.

I only get a few spurts of steam and then it quits. I cleaned the nozzle with wire but still no steam.