Can't find the heart beat on my doppler

Brittany

So I purchased a doppler few weeks ago, took me few days to find it but I did :) baby was down low but I got to hear it everyday. Saturday (2 days ago) we heard it perfect baby was still low I'm only 10 weeks 1 day. This evening nothing couldn't find anything, we looked for probably 1/2 hour came back hours later with belly full and nothing still... worries me just a little is this normal?