Mine wasn't working, either, and didn't improve when I changed the interval. I ended up using Distill for Chrome and setting it up to watch the Studies page.I noticed that Prolific Assistant hasn't notified me in over a month about surveys.
I am still a Prolific member because I caught 2 surveys yesterday when I signed in and did them manually.
Just though it seems Prolific Assistant hasn't worked for me in a month, plus I get 503 errors.
Any info on how to change the settings to over 1 minute intervals?