Hello all I've seen a few different solutions for this well know issue that Slingbox won't fix but I think I have found a more clear solution. Its not actually a fix but it keeps working for me so try it and let me know whether it works for you or not. This has only been tested with Android and Slingbox M1. It seems the app attempts to detect if a Chromecast is on the network and only then displays the casting button. If the app has left over cache it will get stuck and the casting button won't display.
Previous fixes are attempting to overwrite the cache so the app can display the casting button but we can avoid all that with these steps. Keep in the again this is only based on Android and I've personally only tested with a Samsung phone and tablet and it works on both.
Step 1 - Open Slingbox app
Step 2 - Press Home button (main button to go to the main Android home screen)
Step 3 - Hold Home button for 3 seconds then Close and/or stop (bottom right) all apps, press Home again
Step 4 - Hold Home button for 3 seconds then go to Task Manager (bottom right) to clear RAM memory (do it twice)
Step 5 - Repeat step 3
Step 6 - Repeat step 1 and after 3-10 second cast button should appear.
If it doesnt work please post what device you have.