Asumi is already a top-leaning switch, her barrier to topping isn't really skill, just self confidence. She's had a lot of sex with Nanao, so she's very comfortable being intimate with her now, nothing like the awkward fumbling of their first time together. That gave her the confidence to take charge when she felt like doing so the last time they fucked.
Asumi doesn't have that kind of experience with Mai (yet), plus she's been building up the reunion with Mai in her head for years, so she's definitely lacking the self-confidence to top still.
She's struggling to top Sayaka during their "training" for the same reasons, plus in Asumi's mind Sayaka is probably like, the Ur-Top or something. If she can learn to conquer Sayaka then Mai will be a walk in the park.