Hello Healthappiness,
Thanks for asking the Apple Support Communities for help with your Apple Watch. Exercise credit is based on different factors.
1. First, this article provides some information on accurate readings: Get the most accurate measurements using your Apple Watch - Apple Support
2. If you met those requirements and still have issue, make sure you are wearing your watch correctly. These two articles talk about wearing it, and the things that measure your heart rate affect other measurements as well:
Wearing your Apple Watch - Apple Support
Monitoring your heart rate with Apple Watch - Apple Support
3. If you need more help, the next steps to calibrate your watch may be beneficial: First restart your Apple Watch: How to restart your Apple Watch - Apple Support
4. Then restart your iPhone: Restart your iPhone, iPad, or iPod touch - Apple Support
5. Once this is done and the devices power back on, if your devices are not paired, use this article. If your Apple Watch isn't connected or paired with your iPhone - Apple Support
6. Then calibrate the watch using this article: Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
7. If you need more help, please contact Apple Support: Contact Apple Support
Thanks and have a wonderful day.