Most probably your pool.
The amount of blocks it is getting and time intervals between blocks, total pool hashrate the number of workers, your share rate, etc.
E.g.
If difficulty the same but you are getting less... and your pool is getting the same amount blocks - it's probably because there are more miners or higher hashrates taking more shares of each block reward or your hash rate has dropped or your rejected shares have increased.
or...
If difficulty the same but you are getting less... and the pool hashrate is the same, check that your pool is still lucky and getting the same amount of blocks at the same time interval.
etc etc.
You have to know your pool well to understand what is happening and test other pools if you see that you are not getting what you think you should.