hCG not doubling

So

I’m supposed to be 5w5d and these were my hCG levels:

27/10 : 2,348

29/10 : 4,157

31/10 : 6,556

So my hCG is not doubling but is increasing. Haven’t had my bloods taken since the 31st October and due an ultrasound tomorrow. Doctors say pregnancy could still be viable but they aren’t sure.

A gestational sac was spotted on 27/10 but too early to see baby. I’ve had no cramping or bleeding etc, no symptoms of an ectopic either, I feel absolutely fine in myself.

Anyone had any success stories with slow rising hCG?