Hi.
Go to CSF, then go to "LFD BLOCKLISTS", look in there the blocklists that you are using.
Check if the IP is blocked by any of the BLOCK LISTS that you are using, to do that go to /var/lib/csf and search the IP on all the /var/lib/csf/csf.block.NAME black lists in there.
The next step would be to REM the blacklist in CSF BLOCK LISTS, then you will have to delete the list info in /var/lib/csf/csf.block.NAME and the last step after you delete the block list is to restart CSF.