All videos perform for only a certain time period. They get shared and watched more initially than later. Once the shares slow down, views and earnings follow. On average, the first few days are the best for most videos. But if the video gets a good response, it can go for a few weeks. Having great earnings for more than a month or two is not common for most of us.
But don't worry, they can pick up again at any time. I've had videos perform very poorly in the first few months but get 500,000 views in a few months much later on. It's the biggest argument for uploading frequently. Once your channel grows, something will almost alwways be earning you money. Many of the viral videos out there are actually more than a year old when they get traction.
Internet relevance is random and unpredictable. It is also often delayed.