Normally, XP points and distance gains are added to your profile in game based on the distance you ride, but there's currently a display issue on iOS where you won't see this added if you do multiple rides in the same game session. This is a display issue only; progress is still being added to your account. This issue may also be happening on Mac and PC, but we have not verified it yet.
We're working on fixing this issue in an upcoming game update.
As long as you close Zwift from your background apps at the end of your ride, you should see your progress advance properly on subsequent rides. Instructions for how to do this are on Apple's support site.