I've answered a lot of questions, sometimes my answer is the only answer, but the OP has never come back to acknowledge the answer. Forgive me if this is already in place, I have no idea, but the following might encourage OP's to check the site periodically for answers.
When a new member signs up, one of the things that should be presented in bold is that if they ask a question, they should not abandon it, in other words check back periodically for answers and acknowledge them.
The site could send an email reminder if it detects that the OP has ignored their own question which has answers.
After a predetermined period, if still no response from the OP, the OP's should be put on some kind of restriction from posting more questions.
Some people simply do a one time sign up with a throw-away email just so they can ask a specific question just so that they can get a specific answer and once they get their help just probably would never come back so I don't know how useful putting the OP on an asking restriction would be. Instead, OP needs to have a certain amount of reputation to ask a question.