I wouldnt worry, just give it some time. 6 weeks can be too early. I personally had a pregnancy where we couldnt even find baby until almost 9 weeks. All my others were around the 7th weeks mark.
Good Luck
Sometimes you can see heartbeat at 6 wks but it isn't expected until 7 wks. Try to relax and wait, i know that is difficult but all you can do. Are you having another scan next week to check? I had one every week til we heard it which was nice.