I cant find a file that I know should be there

The Files tab only display files that were added to a network or group once you became a member of it.

To access files that were created within networks and groups before you joined them, go to the network or group homepage and select files from the vertical menu to the left of the network/group feed.  This will take you to a list displaying all the files that have ever been uploaded into the network/group, including files that were uploaded before you became a member.

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