The revenue is related to your own hashrate and the network difficulty.
First of all, check whether your hashrate is normal;
Secondly, it depends on whether the network difficulty is rising. When the market is good, the number of mining machines increases, the network hashrate increase, and the corresponding mining difficulty is also increasing. As a result, mining revenue will decrease day by day. The duration is subject to market conditions and can last for several months. And when the market is not good, more coins will be mined.