Asking a question prompts a response. Not only that, but it forces them to take a look at your work again! Below are two screenshots where I have used this technique.
Common questions you could ask in response to a comment are:
What is your favourite part?
Do you think anything needs improving?
Is the idea behind it clear enough?
When you finally do receive the feedback, be gracious. They are not obliged to give you their opinion of your piece. If you act like a prima donna after receiving a little bit of feedback (which you asked for), then you have a lot of growing up to do.
Remember to thank them both times for their time as well! A little politeness can go a long way.
My reply to *travelgirlxx sparked a whole string of conversation between us, and she provided me wonderful feedback!
They've left you a comment, so why not return the favour? If you find yourself frustrated by their lack of wanted feedback, don't get angry with them, comment on one of their pieces with the same level of detail that you would expect. Basically, give what you want to receive.
Some example comments could be:
"Perhaps the eyes could be drawn better but the rest of the detail is lovely!"
"I'm not sure that the blurriness of the text adds to this, but it's really well made nonetheless."
"I adore the rhythm of this piece! But it seems to slow down in the middle, is that intentional?"
Always try and point out a part of the piece which really blows you mind. Are the eyes exceptionally beautiful? Is it framed expertly? Are the colours beautifully saturated that it all comes alive? Or quite the opposite, are they soft and calming?