it can't hurt to wait a lot of doctors believe 72 hours doubling is just fine. and as a matter of fact, mine took more than 48 hrs to double.
perhaps it's a chemical pregnancy. you have to confirm with an OB.
It's been a while since you posted- how are things going? When hcg numbers don't double, it can be an indication that things in the pregnancy are not going well. However, I agree with you- it's best to wait and see at this stage. I have known pregnancies that have not 'looked' good numbers wise and with the ultrasound, but turned out to be fine- unless there is evidence of an ectopic or something, it doesn't hurt to wait and see before taking action.
Let us know how you are doing- I hope that you were able to get some good news!