What each reason means and which ones are worth retrying.
A source that could not be added shows as Failed with the reason on the row. The reasons fall into three groups, and only one of them is worth pressing Retry on.
Out of space — you have used the characters your plan includes. Retrying will not help. Delete sources you do not need, or move up a plan; the row tells you how much you have used and how much you are allowed.
We could not reach the page, or the site refused us — a temporary outage, or a site that blocks automated readers. Worth one retry. If it keeps failing, copy the text into a Q&A or upload it as a file instead.
There was nothing to read — a page that is all images, or a video, or a PDF that is a scan rather than text. Retrying reads the same nothing. Type the important part in as a Q&A.
Something went wrong on our side — worth a retry, and worth telling us if it keeps happening.
Reading a page usually takes a minute or two. If one is still pending after an hour, use Retry — it will be queued again.
Yes. Owners and Members get an email summarising anything that failed, so a source that quietly did not add does not stay quiet.