12.4w can't find HB

I have a sonoline b Doppler I've use for my other 2 pregnancies and I have been able to find the HB as early as 9w before. I'm 12.4 today and still can't find it this time. Last time I saw my RE was at 10w and using u/s they heard it as 174bpm. It's been weeks and everything I've read said I should be able to find it no problem now at 12w. I don't have another apt for awhile should I be worried? I do remember my RE said the baby implanted very high and had to push hard on the u/s wand to even find it at 6w. Could that be why? I thought that may make it easier to find being high in my belly for using a Doppler. Please any advise or if I should be worried.