Hashrate and network difficulty are the two leading factors that impact mining profits. Should you discover that your reward earnings are decreasing, the following factors may be the problem:

To begin with, check the hashrate, staled rate, and rejected rate to look for any abnormalities. Typically, these are brought on by poor connections and hardware issues, and therfore must be repaired in order to improve hashrate or to decrease rejection rates.

Next, check to see if the network’s difficulty level has recently increased. When the overall number of miners and network difficulty are increased, this can result in a decrease in rewards which may last for as long as a few months. The opposite will lead to mining rewards increasing day by day. 

While rare, it is possible for mining rewards to become unstable. For Bitcoin, which uses the PPS+ payment scheme, rewards will also depend on transaction fees (two factors mentioned above excluded). Any “bonus” in your rewards records is part of your revenue and signifies network transaction fees. You can verify the pertinent block explorers for trends in transaction fees.