Whether the lack of success on the question author's part is an indication of this varies by the nature of the question. Ultimately, what determines this is whether the answer remains useful after the author ceases to acknowledge it as such.
Sometimes the author's specific problem isn't the only manifestation of a given question. This is especially the case of a question that has multiple solutions - the author may simply be unable to manifest the reflexes necessary to perform a certain kind of jump, or have item restrictions that prevent a particular route. However, if other people can use that solution to get past the same problem as the author, then it's absolutely worthy of upvotes.
Sometimes, though, the answer simply isn't correct for all cases. In that case, whether you want to upvote out of sympathy for trying is entirely your decision. However, keep in mind that the ultimate purpose of voting is to indicate useful content. An answer which is incorrect but which leads to a correct solution is useful, but one that is simply incorrect is not going to be useful.