Hard to find heartbeat

So im 16 week's, not big by any means i do have an anterior placenta but even my ob today said its just weird that she cant find baby easily. Every appointmentshe has had to bring ultrasound in because we cant find it. Todays appointmentshe said she was going to be more patient and keep looking well we found it twice and then it was gone whxih she said that baby was movinf then after about 5 minutes she found it and was 155. Its just so strange. Makes me feel like somethings wrong. This is my third baby and i have never had this problem. I get every pregnancy is differentbut its just weird.