An uploaded file goes through reading before it is available, and most of the explanations sit there.
It is still being read
A document is processed after it arrives rather than as it arrives. A digital file is quick; a scan or a photograph takes longer because the text has to be recognised before anything can be taken from it.
Reloading after a short wait is the first check.
The file format
Statements and documents are read from the formats the upload accepts. A file in another format is stored but nothing is extracted from it.
Where a bank offers several download formats, the one intended for accounting software reads most reliably — see uploading a bank statement.
Legibility
A photograph taken at an angle, in low light, or of a folded page limits what can be recognised. A flat, evenly lit image of the whole page reads far better, and re-taking it is usually quicker than correcting the result.
Where it went
Uploads land in the area they were started from. A document added under tax documents is in tax documents, not in the media library.
Where the destination is not the expected one, the file is generally present in the other area.
Size
Very large files take proportionally longer. Multi-page statements are read a page at a time and appear in full once the last page is complete.
If it is still not there
Tell us the file name, where it was uploaded, and roughly when — see getting help.