We use the GPS in your BlueLink Hardware and your truck's odometer to record IFTA, so there are a few reasons why they may not be calculating perfectly.
1. The Driver isn't using the AOBRD/ELD correctly. If a driver isn't actually connecting to the AOBRD/ELD hardware through bluetooth from their mobile device or table you will not receive IFTA miles for that driver. This is because without connection to the hardware we have no way of receiving the GPS locations. Inside their app, have your driver go to Settings > AOBRD (or ELD) Connect and connect to their Bluelink hardware which should be attached to the diagnostic port of their truck.
2. Your Bluelink AOBRD/ELD hardware wasn't receiving GPS signal for a prolonged period of time. The blue Bluelink hardware is very good at receiving signal but there are some situations where it may not be able to receive one if it sufficiently blocked. Make sure your AOBRD/ELD hardware is positioned where it can best receive cell signal.