Low HR on Broadcast mode

Good afternoon 

can anyone help please?

when I use HR Broadcast mode it shows my Hr Rate really low and not a true reflection of how hard I am working .

for example it will say 90bpm but if I was using it in run or cycle mode I am 150-160 upwards:

I am using the watch to link with my garmin 830 and it picks up the sensor fine but I don’t understand why the broadcast function is not working .

can anyone help please?

thank you 

matt :)