Miner hashrate, network difficulty, payout schemes and luck value are the major factors that affect mining rewards. If you find that your rewards are decreasing, the following reasons may be culprits.
First, you should check the hashrate, staled rate, and rejected rate to see if there are any abnormalities. Usually, these are mainly caused by poor connections and hardware issues, so you must fix them to improve hashrate or reduce rejection rates.
Second, you should check if the network’s difficulty has recently increased. When the number of miners and network difficulty increase, this may result in decreases in rewards which may last for months. The inverse will have mining rewards increase day by day.
In special circumstances, mining rewards may become unstable. For coins that use the FPPS payment scheme (BTC), miner rewards also depend on transaction fees (excluding the two factors mentioned above). The “bonus” in your rewards records is also a portion of your revenue and represents transaction fees from the network. You can confirm the relevant block explorers for transaction fee trends.