Why aren't my head commands working?

If the head commands doesn't work on your Java Minecraft Server, please check the following points:

Ensure command blocks are enabled on your server:
Within the server.properties file, the following line has to be set to true: enable-command-block=true
Restart the server after you changed this setting.
If you are only getting Steve heads, try /minecraft:give instead of /give. Sometimes, server plugins conflict with the /give command.

Ensure you're in operator and creative mode to be able to use command blocks correctly.
Ensure you used the command in a command block! Custom head commands are to long for the chat and will not work.
Ensure you're connected to the Internet. Skins can't load from the Mojang skin server if you're offline. In very few cases, the Mojang skin servers are down. In that case, you'll not see any skin on a player or any head in game properly, too.
Ensure the version of the givecode match the version of Minecraft you are playing in.
If you're in solo player and would like to use command blocks, press "Esc" and "Open to LAN" in order to turn on creative mode commands.
Ensure you don't use any Minecraft launchers or mods which modify the model of the player (for example TLauncher).