From the picture it does look more like a sugar apple to me, and if you planted an atemoya seed they aren't supposed to be true to type, which is why they are usually grafted. The one I have is double grafted-sugar apple rootstock, then cherimoya, then atemoya. Apparently there can be some degree of self pollination in humid environments.
I found this on pollinating.
How To: Cherimoya Pollination - Tropicals Forum - GardenWeb I wasn't this thorough and what I'm doing works to some degree. I just try to remember to do so twice per day and force open flowers that seem larger in the morning if they aren't open yet. Don't know if that makes a difference though.