The VA is not obligated to review every file for every possible claim when you file for one thing or another. Back then you filed for a knee condition. Had the evidence about your knee revealed, for example, an ankle injury that occurred at the same time, or a scar on the knee, then they were probably obligated to consider the related issues. But when someone files a knee claim, the VA has no duty to review the file for anything else and ask you if you are still having a problem with it. They will very rarely give you something you don't ask for. A veteran can have a file full of all kinds of medical problems, but the VA will only grant service connection if the vet files a claim for it and produces proof that the problem is current.