Morphy Richards Jet Stream elite not steaming?

The problem isn't the same as the other one I've seen asked about in the forum.

I've replaced the anti scale filter and the iron makes all the right noises but not steam is produced. I've put the old filter back in and it works OK. Any thoughts on what might be causing the problem and how to resolve it would be appreciated.
I held the steam trigger in for a couple of minutes after fitting the new filter but that didn't sort it. The lights on the display don't indicate any error.