how to ask help from an area expert and getting feedback?
Start with a summary. Write a couple of sentences to explain the reason for your email and a rough statement of what it is you'd like help with. If your message is long and takes a while to get to the point then the expert is likely to get bored and lose interest.
Explain how you know of their work, and how it relates to yours. As @user3624251 suggests in their comment, a bit of massaging their ego will not go amiss, as long as it is not excessively fawning.
Be polite, but to the point. If you are too apologetic then you'll just come across as underconfident and unimportant. 
State your problem as succinctly as possible. As you would on SE, try to make it as self-contained and well-defined as you can.
If you can't describe the problem in a couple of paragraphs or less, and/or the reply would require more than a few paragraphs, then it might not be well suited to an email enquiry and you might be better off trying to arrange a meeting. You could try finding out if there are any conferences coming up that you could meet up at. Or, if you can sell your problem as being really interesting, maybe the expert would be interested in collaboration.
Accept that, as you have already discovered, you will not have 100% success rate. However well you write the message, you may not even receive a reply, and that is not necessarily a poor reflection on you. But there is very little harm in trying, and some people turn out to be very generous with their time and advice.
Always follow up with an appreciative email if they reply in any way whatsoever!