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.