Reasons for heart rate monitors not registering pulse well?

Just wondered why a persons pulse does not register very well (if at all) on an HRM. I’ve tried several: chest, wrist, and the HRM’s on treadmills and bikes. None of them seem to register anything except for the chest-type monitors; and even they sometimes do not register or are way off. Is it possible the pulse is just too weak?

If the strap doesn’t fit properly, the reading won’t be accurate. This is often a problem for women in particular because most straps are designed to fit a man’s chest. Special sports bras are available to combat the problem; a wide hem is designed to hold the transmitter.