odds on there's an air lock in the system. Modern methods may vary but it used to be start ftom the top of the water system and loosen the hoses in turn releasing any air. You might get away with trying the highest heater hose first.
btw Especially if it's a diesel it can take 10 or more miles to warm up in the recent cold weather. During the recent very cold spell I drove 20 miles, albeit at less than 30 mph, and the temp gauge barely moved.
Right or wrong I reckon it takes 20 to 30 miles to properly warm up an engine in summer
