Why is my latest exercise not uploading to my app

There are countess of possibilities. Without you providing any details, it is hard to advise you something reasonable, but for the starting have a look at this thread, where there are listed some basic issues and their fixes: [https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-mobile-andriod/245295/my-steps-are-not-showing-up-on-the-calendar-on-garmin-app-in-the-watch-fenix-3-hr-the-history-is-still-showing-my-steps-correctly/1162470#1162470] 

... in your case, I would start with the #4, and try importing the respective FIT file of the latest activity (from the folder //GARMIN/Activity on the device) manually at [https://connect.garmin.com/modern/import-data] and see whether it returns an error or not.