vaillent 824 6 radiator system?

first five radiators work ok, 6th radiator only gets half hot which runs from the fifth radiator on 15mm pipe, the hot water enter at the top of rad through thermoststat full open air bleed otherside at top, after bleeding the bottom half still cold. have a room wireless thermostat on the far wall from this radiator, the system alway serviced remarks made it takes time for the return system to work fully, my own thoughts air lock or room thermostat problem. please have you come across this type of problem and how to rectify thank you euan