central heating Works but no hot?

Central heating works, boiler on, power on, all ok from tank (system mate) there are 2 pipes 1 label hot water to taps and the other label is cold in from mains they appear to feed through a mixer, this has a white plastic top which turns + and - ( i think it has the word oventpod written on it) the pipes from this feel very hot and go into pipes that runs up/down the wall ( i expect taking it to the taps) but part way down the pipe it is lukewarm then cold - anyone help ???