Depending on the make of brush, it is usually fairly easy to clear a blocked jet. If it has external jets that have hose pushed on to the rear of the jet then pull off the hose from them (a little boiling water from your flask will help soften the pipe) and turn the water flow back on and it should push the blockage out of the jet hose.
With regard to pole maintenance the main thing is to keep the pole and hose clean. Most of the dirt that gets inside the pole comes from the pole hose. After every use of any telescopic pole I always wipe the hose down as I coil it up to put back in the van. The difference that this simple thing makes is considerable.
Every so often it is worth cleaning the inside of the pole. To do this undo all of the clamps and lay the brush head on the ground (bristle side up) then direct a hose jet of water down the base of the pole flushing out any dirt out, spray the jet in and around each clamp as well, then dry the pole off with a cloth (and a little non-waxed based household polish if required). Some people do the above every week as a matter of course, doing so will prolong the life of the pole as generally dirt and grit are the main wearing agents.