Heart rate

While swimming laps in the pool, using the “pool swim” selection in the Workout app, I attempt to check my heart rate (bpm) on the watch and it is almost always “grayed out”. When the swim has ended and I go to the phone’s Activity app to see the heart rate chart, it displays “chart unavailable “. PoolSwim is the only activity that gives me this problem. The Activity app, on the phone, does display total time, total distance, total calories, and average heart rate (but not the high or low).