same issue here..... everyone on YouTube and forums always seem to have clean microfibers, even if they are years old. I always get some stuff stuck in the fibers, which even with a brush is hard to remove. And I'm surely not going to remove them with needle nose pliers.
Consequently I've thrown out all my long nap microfibers (stuff trapped in there stays trapped) and kept only the ones with the shortest piles which greatly reduces the stuck particle issue.
I used normal detergent, special microfiber detergent and sport clothing detergent. All work fine, sport clothing detergent and special MF detergent are better.
I find that sometimes the dryer kicks out some dirt of the towels that has been lingering around after washing.
I used to use high quality cotton towels years ago, they tend to get hard after a while, but they never have particles stuck in them, they always come out clean. And I mean clean.