If it's added in a module (most probably), e.g. the Custom HTML module, you need to enable plugin parsing there. In the Custom HTML module specifically, make sure the "Content prepare" option (under the "Advanced" tab) is enabled.

If it's another module, it may have a similar option like the above.

Don't use spaces for starters:
Then kindly read the documentation page, it's important: www.joomlaworks.net/support/docs/simple-image-gallery