Washer problems

[flovalflyeruk]

Morning ladies and gents, I hope you can help me with a small but annoying problem. My windscreen wash pump activates ( I can hear it...) but does not spray. The bottle is full and the jets are clear. My father has muttered something about the return valve being blocked. Could this be it and what do I do about it?
Hope somebody can help.....
Cheers,
Flovalflyeruk.