Feel free to experiment with finding a position that is comfortable for you and works best to allow saliva to reach the pad. Gently moving the collector pad around in your mouth can help saliva to reach the collector. You may also try to use your tongue and cheeks in different ways to help bring saliva from around your mouth toward the collector pad.
If the collector pad should fall off, don't worry. Take a new inne STRIP and retake the test. Now, you will have a better idea of how much you can interact with the collector pad. You will get the hang of it in no time ;)