If it is a pumped system first check the pump is working.Assuming it is then balancing the system may be the problem.Gradually close the return valves on the hot radiators( The valve usually has a cap which simply acts as a cover) you will need to use the cap from the inlet valve to do this,as it will slot over the return valve stem and let you turn it clockwise.Only make small adjustments and allow the system at least 10 - 15 minutes to settle down after making a change.Hopefully you will divert some of the flow to the cooler rads.You are likely to have to juggle the settings on some or all to get it right.Don't try to use the inlet valve to control things,these should be fully open