How can I fix my Apple Watch exercise ring?

You are 100% correct about the cause — Apple Watch only adds Exercise minutes when it detects activity at or above the intensity of a brisk walk, based on a combination of:

• Heart rate
• Pace
• Motion calibration
• Wrist movement

Apple does not allow users to remove the “brisk-walk or higher” requirement for the Exercise ring.

Even if you manually start a workout, the watch still applies its intensity threshold before awarding Exercise minutes.

Make sure your personal information is correct: Get the most accurate measurements using your Apple Watch - Apple Support. Apple uses it to estimate your cardio fitness level and this level is used to define what a brisk walk means for you.

The article states:

Every full minute of movement that equals or exceeds the intensity of a brisk walk counts toward your daily Exercise and Move goals. Your cardio fitness levels are used to determine what is brisk for you. For wheelchair users, this is measured in brisk pushes. Any activity below this level counts only toward your daily Move goal.

I haven't experienced that. I walk gently at 12 minutes per kilometer with an outdoor walk workout. It records all exercise minutes. What kind of pace is recorded in iphone Fitness app for your workout?

All ithat minutes needs for an outdoor walk workout is the clock and gps movement. Wrist movement is only required for detecting outdoor walk when you forgot to start a workout. Or as a prior fitbit user, Steps, which can be detected whilst knitting or typing, hence Steps are given a back seat.

It is calories that use the combination of heart, weight, age, time and distance.