If all you have done is fit a new bottle then I would say you have an air lock. You need to follow the bleed sequence to get it out. It can be a pain but honestly it's the only way. I changed a rad not to long ago and had the same problem but after messing for a while with the bleed screws all is good. Also do NOT fill your system through the header tank as won't work. Fill through the cap next to the thermostat.