Not finding the heart beat with doppler at 12 weeks...?

I went to the dr at 10 weeks and they tried finding the heat beat with the doppler...they couldnt find it so i started crying, lol (im a little hormonal). I told her that i wanted an ultrasound bc i had a missed miscarriage before and didnt find out the baby had ped away until 3 weeks later. She told me ok, and they found the heart beat with the u/s and said everything looked ok, except the placenta was covering my cervix but that would probably fix itself. So, she told me to come back in 2 weeks instead of 4 (im not really sure why) and so now i will be 12 weeks. If they cant find the heart beat with the doppler this time, then what? Do i ask for another u/s? I really want to make sure everything is ok, but i dont want her to get frustrated with me. Why wouldnt they be able to find it at 10 weeks, or even 12? Im just scared bc this is my 2 pregnancy and will be my first baby.