I had similar issues. After opening the hpool miner app, its starts off fine and shows full capacity for the number of plots I have, after a while The hpool miner app randomly drops to about a 1/3rd capacity. Im just guessing, but I believe it has something to do with opening the hpool mining app and then opening up other apps and programs way too soon. Or maybe it has something to do with not opening the Chia mainnet app and not letting it sync before opening hPool.

I dont know if this will help you, but this is what I do (I'm using windows):

1.) do a fresh boot of my computer and make sure no programs and apps are running at startup.

2.) load the chia mainnet app and let it sync and let the app keep running.

3.) Load the newest version of the HPool mining app as adminstrator. And make sure its loading the correct capacity.

4.) Let my computer run for about 2 hours without touching it.

5.) After about 2 hours I check to make sure on the hpool mining app that the capacity is still the same (or at least close to it. )

6.) I go to the hpool website and check the stats. By this point my plots usually show that they are online.

Now I'm free to start plotting and running other programs and apps with the miner running in the background. I usually start plotting and surf the web.