looks to me like a nute problem and boyznus is on the ball they look to me to be a bit slow >is it possible that you are over or under nuting or if they are as far as you say then dont worry about it it will just be the nitrogen breaking down as a nitrogen deficiency will cause the burned tips and the yellowing of the leave and this does start to happen in the last week or 2 but as i saythey look abit slow for 6 weeks but still look nice great job my freind . hope you enjoy.the postman. ps we all start to worry in the last 4 weeks of the plants life cycle as we dont want to loose the delisous bud keep and eye on them .