I spent 3 hours today painting trim in my house. I'm using Benjamin Moore Advance and a 2.5" Purdy Nylox glide brush per this article's recommendation. At first the brush and paint were great! But after about 2 hours, the brush really began to lose its stiffness. Instead of a nice chisel shape, it took on more of a 'sweep' shape. See photo below.

This was really hard to work with since the bristles just layed down and therefore weren't getting into the detail and into the corners well.

Does anyone have any idea why this may have been happening and what I can do to avoid it?

Thanks.