Not exactly sure what you mean by the 'barbules falling off'. Is that after or while you are tying? If you are using feathers from one of those little bags, I'm no surprised you are having issues because the quality of those loose feathers is usually pretty bad. From a skin is much better. Just off the top of my head, one solution if you want to muddle through with what you have is put the barbules in a dubbing loop. Keep in mind it does not take much.

There are other silks out there to replace Pearsall, but tying thread and floss are reasonable substitutes.