how to fix?

my potterton boiler fires ok first time but does not heat up water or radiators. once it reaches the temperature it does not fire again until it cools down abt and hour or so. the pump is working fine. i observe that the pipe from the pump to divertor valve does not heat up full length .is there air and how to get rid of this and why does the air get into the system so often
any clues
Dev