Is it a penalty or sandbox or something else?

I have a domain and website that is 6 months old, started doing SEO on it about 3 months ago. Lately, I started pushing PBN and SAPE hard on it, but it seems like website is stuck at all. Max position for main keywords on SERPs is #30, it never pass that page 4 to page 3 or so, it just seems like it is stuck and cannot move, no matter if I add TF40+ links.

So, do you think it is kind of penalty or simply "sandbox" where I should wait few more weeks to get out of it?

Btw, I am in medium/high competitive niche, currently about 50 extremely powerful (TF35+) PBN/SAPE links on it.
With other domains, I usually rank them without problem, but do not know what is with this domain.

Well you said it yourself, the competition is a bit high.

The site is 6 months old so I dont think its sandboxed.

So, it should be stuck on page 4 to page 15 for some keywords for a longer period of time until it gets even more authority and trust from Google?

I had the same problem, ad 2 times a day a fresh 100% uni article from around 400 words, in around one week you will pas to #11, then i had to wait again. Post articles on the mainpage!

Martin1971
(let me now or this did help)

I would like to know if it helped to post two articles every day in week, like Martin1971 suggested.

The sandbox is a myth.

2 articles every week is reasonable, but it should be something different, come from an expert point of view, and focus on users.

I had the same thing i built out loads of new pages over a few weeks and i one day rolled out of it.

This is happening a lot recently .. I have sites stuck on page #2 #3 or #4 for a long time and not moving no matter what I do (that was not the case last year)

One thing you can check to understand if this is a penalty or a normal waiting time is checking your GWT or GA .. if your daily impressions/clicks in GWT growing then this is a good signal that Google is happy with the site, if they are going down your are in trouble