Why Is My HCG Level Low And Not Rising After A Missed Period, Especially After A Previous Miscarriage?

Hi, after taking metformin treatment, i got pregnant last cycle but at week 6 misscarriage occured. Now, I missed my period this month again by one week but my hcg level is very low. Also in my last pregnency hcg level was very low till week 6 and so misscarriage happend. Why my hcg level is not raising ?