Yeah, probably the body shape. After MIGD-370, I downloaded PGD-621, very similar to Dream Woman (same director), starring Hina Akiyoshi. She's very similar to Ai Sayama in many ways, but less pretty, but still somehow I was able to get into her a lot more. Hina Akiyoshi has a slightly more slender frame than Ai Sayama, maybe that was the difference.
Recently I started to appreciate (or try to appreciate) busty girls like Ai Sayama and Hina Akiyoshi who do bukkake/gokkun, because usually they never do that kind of stuff, so I never watch them (e.g. Julia, Rion). But maybe huge breasts aren't really my thing to begin with.
Hina Akiyoshi is definitively slender (her body is hot, I like it). She's tall according to asiacreen (1m75). If you like that kind of body, maybe you will like Yamamoto Miwako. She's 1m70 and does various type of movies without crossing my acceptable line haha.
I found the following one when I was browsing her profile and I can't wait to see it. I'm not a fan of Kobayakawa Reiko but I enjoyed her presence with Miki Sunohara in one movie so the fact that she's starring with Miwako is good.