How tight is the strap? If it is really tight then loosen it a bit, if it is too loose then loosen it a bit. Try it on different locations up and down your arm, some find it works good closer to the wrist, others further up the arm. In theory you should be able to slip your index finger under the strap, but if that is too loose then tighten it up little.
If the strap is too tight it can restrict the blood flow which will stop the sensor from reading the heart rate, similarly if it is too loose then the LED light won't penetrate the skin properly for a decent reading. Give the sensors a clean as well as dead skin and dirt can build up on it.
If you still can't get a reading I suggest seeing a doctor to ensure you actually have a heart beat as the sensor doesn't work well on zombies or other undead. 😉
