I did what I always do to answers like this, and lost track of myself.
The short answer is down there, in bold.
Obligatory Philosophical Rant
I think this is a great question, and I don't know why it's being down-voted. It speaks to the actual goal of these sites--getting people help from experts. If an expert answers you, then doesn't follow up with an explanation, that's a perfectly reasonable thing to be frustrated about. Obviously, as you suggest, you shouldn't be mad at them, there are lots of good reasons for someone not to respond. But that doesn't really help you.
The best way to approach this situation, in my mind at least, is to look at the fundamental base of Stack Exchange--community. It's kind of a weird topic, and I don't see many references to this particular aspect of it, but I think it's important to remember when working with any site in the SE network that people's actions are typically meant to act on behalf of the community as a whole.
What does that mean? (I promise, I'll get to my point soon) When you or I vote to close a question, we're acting as a representative of the community. When we comment asking for clarification, we're asking on behalf of people who may be able to answer. Essentially, this is highlighted by the lack of a messaging system. There are very few (if any) actions on Stack Exchange sites that highlight individuality. Even posts we do make can be easily edited by other members of the community.
That turns into a really awesome question-answering machine, because it brings out the best in a lot of people. This isn't an awesome example, but if I can write amazing code but I'm really awful at explaining it, I can utilize the community to help build on my answer stub to really round it off.
Actually applying said rant to your situation
The very same principle applies to your case. If someone answers your question, don't think of it as a quick chat with a fellow developer (or physicist, or entrepreneur, or bicyclist, or what have you), think of it as the first step in your "conversation" (quotes since it should really only be a question-answer conversation, which is pretty brief) with the community of members who know the answer to your question. Just because the first one who gave you a hint isn't able to, for lack of time or interest, that doesn't matter, follow through with resolving the rest of the question, doesn't mean the system has failed you.
The best solution here is to do what Rene said in comments above: start a bounty, attract more attention to the question, and someone else may pick up where the first user dropped off. This is the best way to grab the attention of the community of people with answers, who can follow through with the questions you asked of the first representative of them.
If it happens very infrequently, I wouldn't mind as an answerer if a user pinged me a second time to grab my attention--it's definitely true that it can be easy to miss notifications. But I'd just make sure you don't do that all the time. If you find a need to constantly double-ping users, that's probably a good sign that something else is going on, and you might want to double-check what information you're including in the original question, and what kinds of questions you're asking to follow up with.
Another alternative if you're desperate and have tried everything else is to send an open message (i.e., without pinging anyone) in chat and ask if anyone can help, or possibly better-yet, if anyone has advice on finding help. You definitely don't want to overdo that, same with any of these options aside from placing bounties, but if you have tried everything else and you're very polite about it, that can be an alternative. Just make sure you remember that nobody in the chatroom owes you an answer, and you might not get any responses. If it isn't too meta, you might even want to ask first in the room whether it's okay to ask for help on a specific question. Different rooms have different cultures.
If your follow-up questions really are unrelated, then of course, that's a good time to ask another question. As I said before, we look for essentially two-message conversations: a question, then an answer. That's the goal. So if your conversation extends into an unrelated aspect in comments, that's not what the system is made for, and another question is ideal.
