All of the answers that have been given to you are correct. There allot of issues that can effect your return path and levels. These can be install related or plant related. It seems that you are a contractor working in a system that is having allot of issues and they are issues that you can not deal with directly. Here is my recommendation to resolve this issue for your company. Get with the installation manager that you are working with and have him get with the plant manager. The three of you can go to the field in some of the nodes that you have allot of call issues with and inject the preferred modem transmit level in the field at several tap locations in the node and see if the return path and CMTS attenuation is setup properly and that the system return is balanced properly to keep you within the specified window for return transmit levels for the modems.
