You're using an uppercase Customspoiler there. When I tried it all lowercase it worked fine.
Huh, I didn't try that. I am trying it again here to see if it works.
[customspoiler='FAQ']
The inside of the spoiler.
[/customspoiler]
Edit: It just did the button. I really wish I could figure out what I am doing wrong.
Edit 2: Aha! I figured it out. I was putting the words on the next line. If you just put them on the same as the customspoiler it wraps it all in.
I.E.
[customspoiler='FAQ'] Here's the indside of the spoiler.
[/customspoiler]
Edit 3:
For more clarity, I was doing this. (Parenthesis are brackets)
(customspoiler='FAQ')
Inside of spoiler.
(/customspoiler)
When I should have been doing what I just did.
Thanks for your help! Glad I got it figured out!
Edit 4: Nope, maybe not. I still can't put paragraphs in customspoilers, just one line.