First, check to make sure you didn’t overlook entering an answer to a question requiring multiple answers. You may need to scroll down to see the additional answer area.

Second, if you are using the iPad app, here is what has worked for me when I know I have answered every part of the question, and the Check button is not highlighted. (If you don’t use the iPad and don’t understand what I’m saying, you can skip the rest of this paragraph.) Exit the Khan Academy app, then swipe it out of the window that displays the active apps. Then click on the app on the home screen and navigate back to the mastery challenge. You should be back okay, with all previous masteries still credited to you, and the Check button should work okay now. This has also happened to me on Practices before getting to the mastery challenge, and the same fix just described works for me.

Chances are, though, that you are using Khan Academy on the website on a computer. I have not had this problem there, but some suggestions I have are to close the browser window or tab, then open it up again and go back to the mastery challenge. If this doesn’t work, I hope that one of the Support Advocates will see this and take over from here. Or you can Report a Problem using the link in the top right corner of Support Community pages.

One final cause may be a lost Internet connection that occurred while you were figuring out your response. Try reconnecting and navigating back to your mastery challenge.