Line progression?!? Low Beta??

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Need some advice. First test was PM 14DPO, second is FMU 15 DPO and this is PM 15DPO, fourth is FMU 16 DPO. I had a beta HCG drawn at 15 DPO and it was 39.4 which felt very low to me, scheduled for a repeat tomorrow. Has anyone ever had a similar experience? Doesn’t look like my lines are progressing?