Can't find babies heartbeat

I'm 8 Weeks 6 days and I had a scan at 7 weeks 1 day and seen baby's heartbeat. However I'm using a doppler and cannot for the life of me find the babies heartbeat.

I'm in constant worry as it is as I have had 2 miscarriages and a still birth. Could I be having a missed miscarriage?

They do say that 8 Weeks is early to find the heartbeat but on my last I found it at 7 weeks.

Any advice?