Worried about my HCG levels...

Hello everyone,
I hope youre all having a good evening.  I am currently 5w5d based on my LMP.  Hcg results are as follows:
8/17: 11
8/19: 27
8/25: 173
They are supposed to double every 48 hours i believe but my last result fell short of the mark as it should have been at least 216.  Anyone experience this before and had a normal pregnancy?  I'm so worried as I had a chemical pregnancy in June.  Really want this bean to stick!!  Any advice?