I like an inexpensive acrylic fiber brush called Simply Simmons…made in the dominican republic, but available in the major art outlets online stores (Artorama, Dick Blick, etc and in big box stores like Michaels.
Yes Simmons is a very common brand; I had a Simmons watercolor brush when I was a teen in the 70s that I did a bunch of neat works with. Really, what you are using is a plain-Jane brush that can be bought almost anywhere. That's good news.
to get the answer, go to my video entitles Brush lettering, Materials and supplies or my video on my video channel about brush on fabric. Both give extensive description of supplies....short answer: it aint the brush!