It could be two things; bad GPS signal or invalid Start/Finish line placement.

Check for bad GPS signal: Open your session and tap the round “Route” button to open analysis of your session. Make sure the graph and map are visible. Scroll the graph to left, while looking at the map. Do the trajectories on the map go on the track or near it within maybe 20 meters? If the trajectories are all over the place, and they do not resemble the real traveled path, then you have bad GPS signal.
How to improve: You must keep the GPS in clear view to the sky. If you’re using the built-in GPS on your phone, then you must keep your phone in a phone holder – it will not work from your pocket! When using the built-in GPS, the best places for the phone are on dashboard near the windshield of your car, or near the instrument cluster on your motorcycle. If you’re using an external GPS, the best place for it is on the tail section of your motorcycle furthest away from the rider. On a car it’s usually the lower part of the windshield. An external Bluetooth GPS is needed if you want to keep the phone in your pocket or in the glove compartment. An external GPS is recommended for anything beyond casual use of the app.

Check for Start/Finish line placement: Open the analysis same as above, and while scrolling the graph look for the crossing of the Start/Finish line. It is a red line with and arrow. The trajectory on the map should pass through the Start/Finish line, to the direction of the red arrow. If the direction is wrong, or the trajectory goes past the trap, your laps will not register.
How to improve: You can either create a completely new track with correct traps, or make a copy of the current track and edit the traps that are incorrectly placed.