OK, first things first. If you have root rot you should cut away the rotten roots. If there are not many/any good roots left you may want to try Sphag and Bag (
Sphag-n-Bag). If there are lots of good roots left then repot into fresh medium.
Second, is watering. Watering weekly is not always the right frequency. It all depends on your enviroment and the medium it's self. I favour clear plastic pots because it makes it easy to judge watering.
Rather than just watering on a set time-frame you should water when the plant needs it. For Phals you should let them fully dry between waterings. With a clear pot you can check if the medium is dry all the way down. Phal roots will turn silvery when they are dry and go green when they still have enough water. Another way to test is to use a wooden stake which can be pushed down into the medium for at least a few hours and then pulled out. If it is damp you know it is not dry yet.
If the medium is broken down then it will dry more slowly than you might exect. Also it may cause suffocation of the roots because it is so packed arround the roots.
How old is the medium. It may break down quicker if it is kept too damp, but I'm not sure what other reasons it may break down too quickly.