What Does A Beta HCG Reading Of 143 After D And C Indicate?

Hi. I had a d&c dec 20th for fetal demise (6weeks 5 days is what the fetus was) I was supposed to be 8.5 weeks along. Its now been a little over 6 weeks, no period. I had beta hcg drawn on this past Friday it was 150 and drawn yesterday it was 143. This seems to be going very slowly...I thought it was supposed to decrease in a "halving" way every couple days? I have had no cramping or bleeding for 4 weeks. Is it possible that they may have missed some tissue during the d&c? Thanks