I lose the heartbeat all the time when I'm dopplering, they're shifty little buggers you know!!
I just find it amusing now, a bit like a game, cos when I find the heartbeat, even for a split second, then it's gone and I have to find it again. DH wanted to hear the other day, but I kept having to get the headphones back again cos when I passed it over there was nothing to hear, even after I'd spent ages finding it!!
Wouldn't worry about it pet, by now the baby's all over the place, just wait til it hardly has any room to move and you will win the game every time

xx
PS - when bubs has snuggled down in tight, the hb is
very faint, but I'm used to it now and don't panic anymore. I just jiggle him about til he comes closer. I'm quite a mean mummy actually, not letting him get his peace!! xx