Few things, did you check in your menu item that points to your K2 Category List if you set Pagination to Auto, Display or Hide?
Also there is a radio button directly below that is labelled
Pagination results: Hide | Show
Also
You will need to see what the following settings are set to as it might be conflicting with your general display of articles (refer to the items in bold)
Leading count
Columns for leading
Image size for leading items
Primary count
Columns for primary
Image size for primary items
Secondary count
Columns for secondary
Image size for secondary items
Please submit your settings on here so I can see, and hopefully guide you further. If you need further assistance, let me know and I can get on there to take a look for you.
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I can't forget to ask this, what version of Joomla / K2 are you running?
Thanks
Chris