Had this loads of times, its always always the same thing.
ITS THE Y-PIECE.
They usually get bits of leaf or a small stone in them and get stuck in there.
Change it out and you'll be fine.
(If you don't have a spare one, you can buy them in B&Q and Homebase, in the rope and chain section.)
How do bits of leaf get into the Y-piece? I have come to the conclusion that it is when you disconnect the pole, the connector falls to the ground bits of crap stick to is. When you reconnect, the crap gets blasted up the pole until it reaches a narrow point- the Y-Piece. In my experience, the jets themselves never get blocked as the holes are quite big.
Took me quite a while to puzzle that out, but now I always keep a spare Y handy.
Its always the Y-Piece.
Always.
That is all.