From the forum 'Hints and Tips' section of the forum rules:
"When you receive no answer to your question it may be possible that no one has the answer. Instead of posting the question again you can reply in your own message and the topic will again appear on top of the topics listing..."
By convention you can add a dummy comment such as " bump " together with a quick re-phrasing of the question, to push the thread back to the top of the list. This lets everyone know you would still like an answer, and has the advantage to you and others of keeping all the replies together in one thread.