There was a 'difficulty bomb' put in place by the Etherum developers for a number of reasons to do with the recent Byzantine / Metropolis changes. So that has made things a lot harder for miners to earn ETH.
*EDIT*: Link to a thread about the last difficulty bomb. https://bitcointalk.org/index.php?topic=2119950.0

Also, the 'number of shares' you submit is related to the difficulty. A lot of pools set a fixed difficulty which they can change at any time. Submitting 10 shares at level 1 difficulty is the same as submitting 1 share at level 10.

If neither of those things have caused the problem, then I am not sure. I don't personally mine Eth, but that does look like quite a drop off to me.

Are all your cards mining on your rig? Have you physically checked them? Sometimes the mining software is running but some cards have glitched out and either mine at 0 H/s or much lower speeds.

Hope some of that helps.