Forerunner 945 run rpm / cadans data missing

Hello. 

I have a forerunner 945 and I am satisfied with it however suddenly after running, I do not have rpm / cadans data anymore. The line in Garmin Connect is a flat red line. 

i do not have a separate sensor, just the heartrate belt. 

can anybody explain this / help me?

thks. 

regards Roel