Charge Nurse Tips

I'll try not to make this post so long winded. I work at an LTAC and act as a charge nurse on an alternating basis. I enjoy being a resource to my fellow nurses. The issue that I have is with the CNAs...or some of them at least. Yesterday there was a situation at work with one of the other nurses. Her issue also has to do with the CNAs (with them being on cellphones and with some of them having attitude) I totally agree with all of her thoughts but am wondering...as charge nurse...what else can I do. Apparently, all of her concerns have been brought up to management already...and things are still the same. As charge nurse, what else is there for me to do? I admit, I am not as assertive as I should be when it comes to the charge nurse role. When I see a CNA on their cellphone, is it my job to tell them to get off of the cellphone? I mean, if I tell them once...ok...but if I keep seeing them on their cellphone...I'm not really in the mood to deal with the stress of aggravation of dealing with that. At what point, as charge nurse, do you pass it on to the nursing supervisor. I want to be assertive but not aggressive. Give me some charge nurse tips.