Can anyone help me with this?

2.7c Ive been running this since it came out and Ive noticed slowly my mining rate is going down and down over the weeks. yet in someways the speed has either gone up or at least stayed the same - the amount mined in a week is going down.  i have a pair of 1080ti's

as an example:

https://imgur.com/4NUafKn.jpg