If it's not through being lazy or trouble learning for medical reasons (hope you get some answers soon on that), sometimes it takes a little bit more maturity than school age and not all children fit into a neat little box.
I didn't do well at maths, but a couple years after school I did night school because I needed the grade. I also did another qualification at night school that I needed to get into the career I wanted, via Uni. Sometimes it takes a while for children to find their mojo and what they're really interested in.
My brother, who ended up as a mature student at uni (we both went a few years after school finished), had a successful medical career in the NHS, got one CSE at school in German, and that was only because dad said he'd buy him the pair of trainers he wanted if he'd just pass at least one! Brother is smart, very smart but did badly at school. I think he is one of those people who always wants to do things on his terms.
If trying different teaching methods, taking away distractions, talking, encouraging, rewarding doesn't work, then you've not much left. Am sure your child will find his/her way ok, even if a few years behind their peers or perhaps something like an apprenticeship in something they like the sound of. Many options ahead!