Well, it seems like she started composing because she felt a little frustrated that the songs didn't quite fit with her vision when others did them - I think something about M in its early stages didn't sync up with how she saw it? Or was that evolution...
Either way, she must be happy with the compositions others give her and how they 'fit', so she probably doesn't feel compelled to try her hand at it anymore. Though she did compose a few songs on MY STORY. Granted that was 3 albums ago, but still lol.
Since Ayu didn't go to any sort of musical school and has admitted that she lacks that knowledge I think it's a little harder for her to do it than some of us might think. I do hope she goes back to it, even if it's just a song or two. I'd love to hear whatever she can come up with for her next record