The water is sucked up by the triangular flat, transparent plastic 'lid' (with two arrows on it) sitting below the bottom tank.
This very easily clogs up with felted carpet fibres. Take the lid off by sliding down (see the manual). First of all clean the rubber seals revealed, so that there is absolutely no grit adhering to them. Open the circular cover towards the top of the lid, into which you would put the hose if you were using the hand tools. Manually remove any rubbish, then flush through with water until the lid is perfectly clear and transparent. Re-fit and all should be well.
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