After HF21/HF22, busy notification lagged behind signficantly, and this was due to API rate limit problem.
While Busy's main function is using anyx.io instead of api.steemit.com, notification was using api.steemit.com.
But probably due to the amount that needed to be caught up, or maybe further reduced API rate allowance of api.steemit.com, notification function hasn't been working well. 11 days behind in my case.
So notification function api server has been swtiched to anyx.io too.
Full catching up may take some time. If it doesn't happen for long, some past notifications will be skipped.
Hope anyx.io can be funded via SPS. It's in fact widely used by many steem dapps. I've already voted the proposal and here's detail from 
Proposal: Funding for anyx.io API Infrastructure Recurrent Costs