It happens, and while I could convince myself that I really care about someone who drops me without even telling me that they were dropping me, I just don't bother with the bullshit. My general attitude toward people who do that is
literally this, and in my eyes, they are just a waste of my time if I continue trying to ask myself what I fucked up on.
Roleplaying is a collaborative effort, almost like a relationship, and sometimes it takes months to find the right partners for you. And maybe if you were actually doing something wrong, you should re-evaluate yourself and ask why people are leaving you, but if you aren't? Then obviously those people aren't grown enough to tell you something as simple as, "hey for reasons 'xyz' im dropping ur ass.'
I don't even require people to give me a reason, and I think that's honestly for the better. I don't want to be hung up on a partner, just as I wouldn't want to be hung up on an ex-boyfriend because I asked him to tell me what my flaws were. Quinn is right, you'd end up feeling like if you fix those things, then maybe they'd roleplay with you, and that's not a good feeling to have, especially if you still get ignored even after having 'fixed' whatever was 'wrong' with you in the first place.
As for why people drop without a word... I think people have already stated that. I'm not going to get into it, because the last time I did that was when I was 14 or something, and it was because I had just started high school and a lot of extra activities, and well, it was a big change. Not that it should be an excuse for dropping without a word, but I've matured since then and haven't randomly dropped a partner.
The best advice I can give you is to stop messing with those irrelevants and casuals - you don't need a response from them to validate your ability as a roleplayer/writer. If I could easily get constructive criticism from one of my BMR friends, why would I want to get it from someone who literally means nothing in my life except being a 2-second RP partner? Just be like, "effing bye felicia" and move on. Like Jay Z, on to the next one.
Now, I understand being shy or whatever, and that's why I tell people from the beginning that if they have a problem with me or my writing, then no matter how awkward they may feel, then to just tell me. I don't ask questions. I don't get upset. I say, "Well, for what it's worth, I really hoped it would work out. If you ever feel like you wanna roleplay with me again, just let me know." Done. No big. Like Quix, I find it rude and disrespectful to completely ignore somebody because it makes you feel uncomfortable. It makes me feel uncomfortable to tell a dude that I'm not interested in them, but I do it because leaving a person hanging is rude.
BUT, I think there are ways to combat lack of communication and I definitely think there are ways to break the barrier of shyness. Like Quix said, OoC chat is a great way to maintain great communication. A huge thing for me about roleplay partners is how well we get along just as people. It makes things a whole lot easier if you need to remove something, edit something, or just tell them that something's not working for you. And finding those partners takes time. Sometimes it took me a couple days, and even in one case I thought this person totally hated me, and then it turned out after I contacted him again like 4 months later, we actually got along great. And, as an added bonus, if one of those close partners disappears without a word, you'll feel better about the fact that they're your friend, and you won't even mind it half as much.
So with all that being said, I hope everything works out for your roleplay endeavors. Just keep on doing you, let the irrelevants keep being irrelevant, and know that there are partners who actually care about being courteous.