To start a recording please follow these steps:
Warm up your hands. Cold hands can reduce blood flow to your fingertips, making it difficult for the sensor to detect your pulse. Try rubbing your hands together or placing them under warm water for 30 seconds to improve blood flow.
Apply only gentle pressure on the lens with your finger. Pressing too hard on the lens can actually impede blood flow and make it harder to detect your pulse. Try to use about as much pressure as the phone would have resting your phone on your fingertip.
Make sure your finger covers the whole lens with good skin contact. Covering the lens completely with your fingertip avoids any external light disrupting the recording and ensures a good view of fingertip blood flow.