I have the 5 and bought my wife the 5s. We do a cardio workout 3 times per week together and last night she mentioned that her device never logs any intensity minutes. I took a look and sure enough Garmin Connect showed zero intensity minutes after our workout whereas mine reflected 21.
I double checked her activity and it did record her HR so it wasn't an issue with recording. She spent time in all her HR zones during the workout. I went back to previous activities and all of them recorded zero intensity minutes except for one workout but I can't figure out why.
Any thoughts?