Sonoline B tips

Mandy • 29 years old - Married October 2013 - Mom of Baby Girl Dec 2015 😊 Expecting Baby Boy April 2018 ☺️
Just got my sonoline b Doppler last week and we are having a bit of trouble. I'm 14 weeks and I think we are finding the heartbeat - generally around 130. But it was 156 last time we were at the doctor at 13 weeks. Are we doing something wrong?