Heart Rate stops showing proper rate

For the first month of using my Karoo2 I would see my heart rate go up and down throughout my entire ride.  Now, on the Karoo, my heart rate will show up to a certain point, let's say 135 bpm and then not move up or down for the rest of the ride.  This has been happening the last couple of weeks and each time it goes up to a different number.  When I check the analysis in Strava, it shows the heart rate graph and data and my rate could be up to 157, so the Karoo is recording just not displaying accurately.  Any thoughts?  Thanks