What week did you hear heartbeat at?
I heard this one at 8 weeks... but last one I didn't hear at my 12 week checkup. Because I was worried, she had me come in a week later and we found it fine.
At 12 weeks baby is still somewhat small... but has learned to move. If you are looking for the heartbeat when baby is on the move, you could constantly miss it. I wouldn't worry too much yet, but if you're concerned I'm sure you can call and see if you can come in again in a week to check again.
I heard it at 10 weeks, but it took a while to find it and we ended up switching for a stronger doppler to find it and it didn't last too long before the baby must have moved or something. I also had to lay down with my hands under my bottom pushing myself up in a strange position, which seemed to help. I wouldn't worry! I know it varies on a lot of things! Movement, weight, position of your uterus, etc. I'm sure you'll be hearing it soon!
I saw my MW at 10 weeks and we didn't hear anything, but the doppler was "picking up" a faster heartbeat in there. I had unexplained bleeding at 11 weeks and have been able to see the baby's heartbeat (which has been a big relief). We have a doppler here at home and I've randomly tried a few times, to no avail. UNTIL this last Sunday - I put the doppler on and we heard a huge strong heartbeat immediately - 13w6d.
My midwife found it at 10 weeks. She said that she doesn't usually try at that time, but I promised her that I understood that all could be okay even if we didn't find it. Per our 13 week ultrasound, they said that the placenta is laying across my back which is probably why they could easily find it and also why I've been feeling movement quite early.