3
Do you mean that it won't show on the page even though you're sure it is checked to show or do you mean you can't find it to even check?
Here's a little trick to find a missing block, right there in the Group permissions if you click on the block (it is also a link) you'll get the block on screen so you can set it up.
I've "lost" blocks by forgetting which page I set it for (set to Off you forget where you had it last time) and thought I had to visit each one before finding it.