I know lots of reports have been coming in about lag about this, but this is something different. The mobs won't combine, some of them do and some of them don't TannerLittle CypriotMerks
I don't think already merged mobs merge with other merged mobs. (I hope that makes sense. Basically, unless they are singles, they don't merge together.)
Lets say there is a chicken 2x and another chicken x8 they can combine since I saw it lots of the time. It's just something with these 2 chickens, all the other ones work and sometimes some of them don't.