Love mine.
Luckily I know exactly where to put it as it is the only place in my tummy where it feels like a label sticking in (wierd analogy but those who enjoy clothes shopping will get this) however sometimes it is quieter and then comes back louder in a matter of minutes. Assume it is wriggling at that point!!
If you are having trouble, relax and keep trying.if nothing after a little while, try again another time of day. I pick up best after food and after a toilet trip. Close your eyes too as this may help focus your hearing better.
Just make sure you actually use it for its intended use and NOT to use to compare gurgling bellies and heart beats between you and your OH. Which is obviously what I have heard as advice and never done this myself. Lol.