Not true about the missed period. Depends on when implantation occurred as well as everyone's body hcg rise differently. You said it yourself with other two you got positive at 6 weeks. Keep testing every other day until period arrives. Digital also doesn't mean sensitive. Make sure your using a more sensitive test like a 10 rather then a 25. For me a 25 I dont usually show anything until 18 days after ovulation, even then its a vvfl.
How sensitive are your tests? My clearblue digital how many weeks tests are 25MiU which is rubbish! I’d be trying something much more sensitive like a first response 6 days early - 5.5miu
I personally got 3 or 4 “not pregnant test before my first “pregnant” at 7 weeks with the little