Some site owners use frames on their website to help formatting. HTML frames allow documents to appear in multiple views, which may be independent windows or sub windows.
ReachDeck will not appear on sites with frames as there are multiple windows.
To check if a site has Frames:
1. Right click over the site and select View Page Source:
2. In the Source code select Ctrl+F on your keyboard and in the search bar that appears search for <frameset> if you see this value somewhere on the page then the site is using Frames.