Slow rising HCG!

After a miscarriage a few months ago I’m been monitored in my new pregnancy I’m around 5+4 and my HCG levels aren’t rising fast enough. I’ve been having them done every 48hrs for the last week they gone from 410 - 495 -764 -1067 -1348

I had an ultra sound 3 days ago and the sack has grown from 2.5mm to 6.5mm in 8 days but can’t see anything else. I’m back next Wednesday for more blood and a scan. Has anyone had this and had a successful pregnancy? I’ve no bleeding just pain in my lower back and like period pains which have started today.....