First check that the infant car seat is sitting correctly in the Infant Car Seat Adapter. The infant car seat should sit in the adapter so the angle finder of the car seat is showing the right angle as recommended by the car seat manufacturer. If there is no angle finder there are two things that should be considered:
1. The strap from the Infant Car Seat Adapter should go through the belt guide on the car seat.
2. The infant car seat should be inserted into the adapter having the upper bar support the upper section of the car seat and the lower bar supporting the lower front section of the car seat. If the straps are not fitting it is most likely because the orientation of the car seat is incorrect.