HCG didn't double fully

I'm 5 weeks and 5 or 6 days.

I had my beta drawn on Thursday and it came back at 6089.

I had it drawn again yesterday and it went up to 20066.

Shouldn't it be higher by now? If it doubles every 48 hours, I'm 4,000 below where I should be.

I had it drawn at about the same time of day both times. My Dr wants me to go back once more this week.

Should I go in tomorrow or wait until Thursday?