Power Craft Presuure washer not working properly?

We have a Power Craft 7944 Power jet washer which has served us well, but for some reason is now not jetting under any significant pressure. We have tried all the obvious things - tap fully turned on, no kinks in hose, good water supply pressure - but the lance jet is far too weak and is not achieving the results that it should.
Are there any filters that need checking, or other potential blockages? Your help would be appreciated as we want to get on with the job of cleaning a large area of paviers.
Many thanks
Jon.
Tel: 07852 17 61 61