The problem could be either related to your website or PBNs or both,
As well as the links pointing to the PBNs, but less likely the case, unless you're selecting average/bad domains for most of them,
What's your ranking currently?
Are there any new competitors out there?
Possibly some people are investing more than you do,
How about your website content & on page?
Google had made a shit ton of changes in the recent couple of years, EAT & content do matter than before,
How about the PBNs content & on page as well?
If you used to use spintax content or any kind of shitty one, possibly the PBN websites are getting doomed because of this, so is your website in return,
You need to use decent content for the PBNs as well, publish posts regularly, work on EAT, add internal links, etc... Just like any other money site,
Most of the work could be automated today with AI, but you still need to invest in both time & money to get a decent well built one
What type of links are pointing to the PBNs?
Possibly many spammy links used to do the work for the PBNs & boost your website in return,
If they got filtered and currently seen as spam or less valued type of links then your PBN will be less valued as well,
Especially if you're boosting the PBN with GSA links, web 2.0s, comments, etc...
Other than that, it's not a one single problem that will be solved, there's a ton of parameters to check to make sure you have the right links well set,
And even then, it's really up to google to consider what pages/websites are authorities & rank them, they're messing a shit ton of people websites sadly
