Fibricheck is a medical device that accurately monitors the processing capabilities of your phone during a measurement. It is possible that your smartphone suffers from performance issues which can negatively influence the quality of the recording.
You can try to solve this problem by closing down other apps that are running in the background. Often the App store or Google Play store is performing updates on the background, turning this off could solve this issue.
Keep in mind that this test result is not a diagnosis. It does not replace medical advice, please seek professional medical assistance if you are or believe you are suffering from any medical problem.