Fluctuation of mining profit is mainly result from changes in hashrate and difficulty adjustment.
1) Changes in hashrate: The output of mining is a volatile process. The factors of power, network and miner status will influence hashrate greatly, resulting in changes in profit.
2) Difficulty adjustment: The difficulty of cryptocurrency will be adjusted after a certain period of time. Overall difficulty has a tendency to increase. It is normal for miners to have less profits while maintaining the original hashrate.