I only recently realized that I ought to tag the post where someone solved my problem as the "Solution." Problem is, when I click the little green solution check icon, I get a window thanking me for my feedback, says the icon has been removed, but the green check mark never appears on the thread listing for the post that solved my problem. I've tried clicking again, then the little thank-you pop-up changes to one that displays the green checkmark. No matter what state I leave it in, no "solution" mark ever appears on the thread in the forum listing.
The first time you click the solution button it should place the solution on there. Though you might need to refresh the thread or post page to see the solution icon. The next time you click the solution button for the same post it will remove the icon, you will need to refresh the thread or post page to see it removed. This way it acts as a toggle. Please give it another shot and make sure to refresh the post / thread page to see the icon status update.