When a lead has time-based workflow scheduled, such as a future reminder, it can't be converted.
Not our fault, but the BUG here is that this fails SILENTLY - there is no message at all, the lead conversion just doesn't happen.
Need to either display a message, or find a way to delete that workflow before converting. In any case, we should check to make sure something happened to leads when you click convert, and if not, AT LEAST say: "Something went wrong."