The link points to private content
Private accounts, close-friends stories, highlights from a private account, and anything else that requires being logged in to view are not retrievable by any tool that respects Instagram's own access rules, Pullgrab included. If the content asks you to log in when you view it on Instagram, no downloader can bypass that honestly.
The post no longer exists, or the link is wrong
Deleted posts, typos in a pasted link, and expired share links (the long `?stkn=` parameter some shared links carry) all produce the same generic error. Copying the link fresh from Instagram's own share menu, right before pasting it, avoids most of these.
The video downloaded, but the file won't open
This is the one that confuses people most, because it looks like the tool is broken when it isn't. Instagram doesn't always package a video the same way: sometimes it's a standard file any app can play, and sometimes it's a more efficient format built for streaming apps, which a smaller number of basic video players, notably some phones' default players, can't open, even though the file is complete and undamaged. Pullgrab checks for exactly this and automatically retries with a different method when it detects it. If you ever hit this with any tool and get a file that won't play, opening it in VLC (free, on every platform) almost always works, since it supports far more formats than a phone's built-in player does.
Rate limits and temporary blocks
To keep the service fast and fair for everyone, there are limits on how many downloads one visitor can request per hour. If you hit one, wait a bit and try again; it resets automatically.
The account or the content was removed
Less common, but it happens: an account gets suspended, or a creator deletes their content between when you saw it and when you tried to download it. There's nothing to retrieve at that point, on Pullgrab or anywhere else.