The mining yield is mainly affected by hashrate fluctuation and difficulty adjustment.
1) Hashrate fluctuation: Essentially, hashrate output is an unsteady and fluctuating process during mining. The factors of power, network, miner status will also affect the hashrate considerably, resulting in profit change.
2) Difficulty adjustment: The difficulty of cryptocurrency will be adjusted after a certain period of time with an overall tendency to increase. Therefore, it is general for miners to have less mining yield while maintaining the original hashrate.