Why is the income of Dogecoin getting lower and lower?

Dogecoin is merged mined with Litecoin, f2pool currently adopts the PPLNS model to calculate Dogecoin rewards separately. Dogecoin Rewards mainly depends on the difficulty of Doge mining and the actual output of the mining pool.

Dogecoin’s network difficulty is adjusted frequently, As the difficulty of Dogecoin continuously rises, it may lead to the decline of Dogecoin rewards. Moreover, there is no direct relationship between Dogecoin and Litecoin's mining revenue and there is no fixed reward ratio.