No high pressure?

I have a Karcher 411. It is a bit old but has worked very well for years.
Recently, it has started to play up as follows:
When I used to operate the lance, the water would come out momentarily at low pressure and then the high pressure would kick in. It always worked this way from new.
Recently, I had to operate the lever on the lance more than once to get the high pressure to start. This got progressively worse and now the high pressure will not start at all.
The motor still runs but the high pressure does not start.
Any ideas?