My kids never clicked with their cousins. It didn't help that all of their cousins are older them them - the youngest are 3-4 years older then our oldest, and most are 5-15 years older - and don't live close by. Also, my brother's son was not an easy kid to get along with, and him and his sisters are the closest in age. "Timmy" would bully my oldest son whenever the family got together, teasing him for being skinny and for having glasses, and the like, despite being five years older, and his behavior (and my brother's refusal to even condemn it) drove a wedge between the cousin group, and while some of the other (older) cousins are still on 'greeting card' terms, but nothing more.
Its sad, my mother recently passed and none of their generation, save my youngest two (12 Y.O. and 17 y.o daughters) even bothered to show up because of this feud. My eldest (college junior) claimed he couldn't get off work (he's proven very capable at getting off work before), and equally flimsy excuses from everyone else. My father was utterly devastated, as am I.