I have noticed that when i ad new links towards a inner page it quite often makes a jump up, down a lot worse than before and then it starts to climb towards new highs.
As the person above me has written, the quality of the backlink is also important and there are a lot of sites made for selling guest posts the really are linkfarms that doesnt help your rankings or potentially even hurt it. I'm not an expert but this is what I try to avoid when choosing a link:
* Spammy link profile on the site for a potential guest post
*That most articles have outbound links
*That it is indexed and preferably has a bit of traffic
*That the content on the site reads well and doesnt seem like AI-content.
I have gotten away with buying a bad backlink here and there, but when multiple bad backlinks start linking towards a specific article I have seen rankings go down.
Hope this is any help
/Birdiemachine