Not sure how many dpo you where on the second beta but hcg takes longer than 24 hours to double usually.
For HCG levels <1,200mIU/ml, the expected doubling time is 48-72 hours.
For HCG levels 1,200-6,000mIU/ml, the expected doubling time is 72-96 hours.
For HCG levels >6,000mIU/ml, the expected doubling time is >96 hours.
For HCG levels <1,200mIU/ml, the expected doubling time is 48-72 hours.
For HCG levels 1,200-6,000mIU/ml, the expected doubling time is 72-96 hours.
For HCG levels >6,000mIU/ml, the expected doubling time is >96 hours.