When things don't work (as they should), here are some simple things to check:
The plugins folder of your server contains 'Custom Items.jar' or 'Custom+Items.jar'
(downloading it might replace the space in the name with a plus, but that's fine).
The plugins folder of your server contains a file starting with 'Knokko+Core' or 'Knokko Core'.
The plugins folder of your server contains a folder called 'CustomItems', which contains a file
ending with '.cis' or '.txt'.
1 of the following conditions is met:
You installed my resource pack plug-in.
The 'resource-pack' and 'resource-pack-sha1' in your server.properties aren't empty.
If any of the things above is missing, things are not going to work.
If you changed anything, restart the server to test if it works now.
If it still doesn't work, do the following checks:
Is the CustomItems version you are using the latest released version?
If not, replace your current version with the latest version and restart your server.
Is your KnokkoCore version the latest for your minecraft version?
If not, replace it with the latest version and restart your server.
Automatic checks
Use the '/plugins' command and check that 'CustomItems' and 'KnokkoCore' are listed.
Try '/kci debug'. This will work if you are using Custom Items 8.0 or
later and your installation is partially correct.
Use the '/customitems list' command and check that it lists all your custom items.
Check your server console/log, copy it entirely, paste it in the box below and click on the 'Check console' button.
If everything above fails
Post all results from the steps above on the discord
or BukkitDev.