Can anyone help us understand why it's happening like that?
Are there any mandatory columns should be in import file?
We have followed help documentation links to do this procedure. But those documentation links have information till upload only.
Here is the ref link: Create a customer attribute source and upload the data file
What do you mean by not getting the values? Are you able to find the count of [customerAttributeName] metric?
Aside from that the dimensional values would be related to whatever values you already have in your implementation.