If you are a  new user you might fall in problem using your commands. Let me show you the cases of command not working

1. Command Name issue.

If you have spaces in your command name like whats up then your command will not work. Because command names can't have spaces. So wither name it whatsup or whats-up or anything between them. 

2. Case Sensitivity

Custom command names are case sensitive. So if you have a command name Apple then apple will not trigger the command. So you have to rename the command name to what you want or you can alias it see Aliasing a command 

3. Having additional prefix

Command by default have a prefix which is ** but if you make a command named !something then doing just !something  will not work so you have to do **!something. Sounds weird right? But its not. If you go to Settings in the dashboard, you will see that there is a server prefix which is ** . So if you want !something to work then you have to make the command name to something  and change prefix to !  and then !something will work perfectly.

4. Permission Issue

If you have a server where bot don't have permission to send message in a channel you are trying to run command, then commands will not work, because bot can't send message there. So you have set permission in the channels so bot can send message there. 

I think thats all. If all of above don't help you. let me know in the comments I will help you out.