Some years ago I attended a master class in portrait painting given by an excellent artist who had painted many famous and wealthy people. She was nearly 80 years old at that time and had been a working artist since her early twenties.
A class member asked her about the oil painting brushes she would recommend to us. At this moment she became very agitated and told us her experience with 'new' brushes. 'I will buy new brushes and then begin to use one and it is just worthless. I get so mad I just break it over my knee and throw it on the floor of my studio.'
I can relate. I own many brushes and there have been some that I love at first use which do not hold up. And I know how to care for them. Many simply do not hold their shape, even with the careful dressing I do with my fingers after cleaning them with non-detergent soap. One artist whose blog I have read will actually clip the brush so that its shape is pressed with stiff paper overnight. That didn't used to be necessary.
What can one do? I have sent disappointing brushes recently purchased back to the manufacturer (a very old and well known company) and they replaced them and also told me that the brushes I returned were, in fact, defective. They also sent me some other goodies I could use in my studio. But they did respond and so if you can take the time, don't break your new brush over your knee, return it. I think it is important to speak up and get the excellence we need to ply our craft!