Nested Spoilers Don't Work

Hello all,

I am desperately trying to put nested spoilers into my work and I can’t get them to work at all. I don’t have this option on Shortcodes Ultra but have been adding the manual code:

[spoiler title=”Master Spoiler”] etc
[_spoiler title=”Nested Spoiler”]Hidden content[/_spoiler]

but it doesn’t work. Is it because I’m inputing it manually, should I search for a plugin for this specific feature?
Also I see a lot on here about ‘Compatibility Mode’ under Settings but I can’t for the life of me see it. Can you help with this?

Many thanks,

Angela