Hello _JR__,
Thanks for posting in Apple Support Communities. It sounds like your Garmin app is syncing some data but not the full history. Make sure that your settings look correct from Manage Health data on your iPhone, iPod touch, or Apple Watch - Apple Support which will allow 3rd party apps of your choosing to communicate with the Health app.
If everything looks good there, you may need to check your settings in the Garmin app. We also found the following page via an Internet search which may help - I Am Missing Wellness Data in My Garmin Connect Account.
If you need further help, Garmin Support may be in the best position to assist.
Take care.