16 weeks 4 days

So, a while back we bought a fetal Doppler and we usually find baby's heartbeat right away. Well last night we searched and searched for it, with no luck :( I tried again this morning and still nothing :( what could cause this? I'm fearing the worst 😭
Update: finally found it drank some really really cold water and laid on my back. I just have a very stubborn baby. Thank you for all your advice. :)