Keyboard notes doesn't always respond

Some of my keyboard notes doesn't respond unless I slow down my writing and press them a little bit harder and more individually. It started first with 'A' and then spread to 'S' and now perhaps 'G' and 'T'. None of my other notes are acting like this. I have not done anything in particular to damage the keyboard, I think?

Why is it acting like this and how do I fix it?

Macbook Air 11', not even a year old