Can't find HB on Doppler anymore

I found it with this baby at 8w5d. I tried again last night and couldn't find it until almost three separate times looking and it didn't seem as strong or loud as the first time. I tried this morning and couldn't hear anything. I'm 9 weeks today. I could never hear it this early with my son so I wasn't expecting it this early, but now I'm freaking that I found it once and can't seem to get it again...