music albums splitting on iPhone but not on imac

Hi! I can't find a solution to this and have looked pretty hard. Some music albums split on my iPhone, iMac they're ok. It's a few of my compilation albums that do it. I've tried to check compilation box and uncheck. Delete and resync. All fields in 'get info' are right. I've tried to create AAC versions of each song and replace the old. Hope someone can help. Thanks