It's fairly common not to see it at 6w2d.
I saw it at 6w2d with my DS with an internal scan. But my OB commented "wow this bubs heart must have only just started beating" because that day it was 114bpm. It starts out slow and speeds up over the next few weeks. He also warned me before he checked that we may not see it and that he wouldn't be worried if we didnt. I bet next time you have a scan it will be fluttering away.
Congrats