I didnt pick it up my first day, I skipped a day and tried again. I've done it twice in a day when I was worried. Youtube videos helped me and I also used a good amount of gel which I think helps cut the static and pick up the sounds more clearly. If you know the side your baby is on it kinda helps and even then its tough. Last night I heard the heartbeat but it was there for a few seconds and then shifted slightly to the left and then it was there for another moment and then I lost it. I tried again for another minute and then said screw it, I listened to it for long enough, baby is fine. I had a tiny amount of spotting after no blood for a week so I was nervous and couldnt sleep.
Also, I don't know what your body is like but I am kinda bloated right now and put on over 10 lbs so I push all my skin/fat etc up so the area is taut when I use the doppler, I find it helps.
Sometimes you only hear a glimpse of it for a second. If you get that, try hanging around that area super slowly, barely moving and see if you can pick it up again. You may also be testing on the wrong side. Do you know if the baby is on the left or right?