I found when I used the Doppler very early on, it was easier to find, little baba didn't move around so much. Then closer to the weeks that I started feeling him move (I felt him a little bit around 16 weeks, but proper kicks at 18 weeks) he was very wiggly and moved all over the place from week 14 onwards.
It's fainter depending on where he's sitting and facing which way, then sometimes I'll move it again and it's stronger than ever and I know I'm the closest to his chest.
Sometimes baby might be upside down, right way up, facing outwards, facing onwards with his back facing Doppler.
This is the reason why midwives and doctors hate women buying dopplers, my midwife ranted about it when I mentioned I bought one. I said I found it straight away and rarely used it. I'm a worrier, but I know I'm not qualified enough to know whether something is wrong or not, so I used the Doppler as a 'making memory' thing rather than to check if my baby is still healthy tool. Every baby is different too. I know it's hard not to worry, especially if there is a history, but please try not to rely heavily on the Doppler. If you struggle to find a heartbeat, put it aside, get up and do something, have a cold drink or a snack, then try again a little later on when baby is probably in a much better position and had moved for you
I remember the first time I struggled to find a heartbeat. I had found it perfectly every time before, I just hadn't realised that maybe he'd had a growth spurt and was much higher than usual. Then I noticed he liked to go to the right of me for a few weeks, then he's moved again. You'll notice a trend soon
Saying that though, I feel my baby has always been very very low down. He loves being snug and warm right in my pelvis, even at 22 weeks, he's now kicking and I don't use the Doppler (i admit, I need to buy new batteries as I left it switched on the once) but I noticed he's still very low down, and most ladies I know at this stage have their kicks much higher than mine! So remember every baby is different, maybe yours liked to be very low down too and your pelvis is making the heartbeat sound weaker and muffled?
Please try not to worry
don't sit for ages trying to find it. Baby is most likely asleep and if you go and do something different he/she may wake up and move into a better position for you.
Soon you'll feel the kicks and movements though (or perhaps you've already heard a few thuds on the Doppler!) and you'll get used to their patterns! Although, those patterns will still change. Mine woke me up with movements every morning between 5am-6am. Now he's more active later on in the afternoon the past couple of weeks
There are a few days I worry he hasn't moved much and I do go to the Doppler (forget I've got no batteries for it haha) and then I think no I'm not gona make a mad dash to the shops to buy batteries for it, I'm going to do something for a bit and hopefully he'll wake up and I'll feel him. And I always do.
I would only worry if you feel unwell, in pain, having any loss of blood. Otherwise if you struggle to get a heartbeat and you feel fine, then leave it be for an hour or two, do something different, and then try again!
Hope that's helped a little! I can understand how horrible anxiety and panic attacks are, I have them regularly and they truly suck! You already feel tired from pregnancy and panic attacks will only exhaust you even more! So relax and please do not let your imagination run away from you! Xxxxx
I have felt he/she move lots and lots since then so I have tried not to worry. Hope you're okay 
