I think you need to provide a bit more information before anyone can start to come up with any suggestions. There can be many, many reasons for no paint flow. eg. clogging, incorrect adjustment etc.
It's a generic brush, ok. what type of "name brand" brush does it most closely resemble? single or double action, etc. etc. Have you used it before? If so, has it been cleaned properly? Sounds simple, but everything you can tell us will help.