Today I'm 16+5 and, until today, we've generally heard the heartbeat very strongly on the doppler (maybe 5 times?). Today though we only heard it very faintly and twice - neither time for more than 4 seconds or so.
Now I'm aware at this stage the baby can be moving around a lot and might just be further away and the heartbeat is less likely to be detected at certain times. But at what point was the baby big enough that you could hear it easily every time?