it could be an expansion crack.your building natural moves.or bad craftmanship.or the render dryed out to fast when setting.unibond the crack and refill with render just like polly filling. but leave the newly apllied render to harden and gently rub [not with a float] but a house brick untill even.try a small area frist.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix decorating.