There are a number of causes of shedding of a hair brush. Because of an issue I had with a Simpson brush I read up on and learned a lot about the subject. You can read about it here.
First, in the brush making process, brush manufacturers use a comb to take out any hairs that did not set properly in the epoxy. At times not enough hairs were taken out and the result is shedding right out of the box. It can take a month or two for these hairs to work themselves out. This period of time can be exasperating to the user but usually it is only a few hairs that come out. If the fallout is excessive, the brush may be defective and should be returned. It could be that the epoxy did not set properly and if that is the case the brush will shed forever.
However, the advice you will be given by a vendor or manufacturer is to wash the brush a few times and comb it out after each washing. If the brush is not defective, that will take care of the problem. However, it does not take care of a defective brush.
My advice is to take pictures of the hair loss and show them to the vendor or manufacturer. Write to them immediately so you have a record of your problem. Some brushes come with a limited time warranty which can be extended by the vendor or manufacturer if they want you to try some steps to remedy the issue.
I had my brush for over a month and it shed as many as twenty hairs each time. It never stopped shedding and Simpson replaced the brush. The replacement brush began shedding right away but after a few washings and comb outs it shed only two or three hairs, which is acceptable since the brush is so dense. Now it sheds rarely and then only one or two hairs.
Shedding can also result from improper use of a brush. For example, not soaking the brush in warm water will result in bristles being brittle and may cause them to break off. Soaking brush in water that is too hot can result in the epoxy being weakened and resulting in hair loss. Not rinsing your brush out after use and drying it before storing it can also result in shedding.