Hi Jackknife, i have the same issue with my 100lb fiancee. i went to the dealer several times, we found out this is not a problem and it occurs from time to time if the passenger is under 110lb.
the solution me and my fiancee found for ourselves is the following procedure.
she seats normally and if the ab does not recognise her, she puts her back against the top part (back) of the seat without the seat belt, then she seats at the really back part of the lower part seat pushing a little bit harder downwards (like increasing her weight). in adittion, she always seats with her bag on her laps and then she quickly buckles up. the ab is on and then leaves her bag down by her feet. this works for 99% of the cases. if it doesn t work, i turn off the engine and we try again. but as i said, reseating with an added pressure to the seat almost always solves the problem.
(sorry i don t know the terms in english. if something isn t clear, pls let me know so that i try to describe again)