Home Doppler?

I've been using my home Doppler since like 13 weeks and I can always find his heartbeat pretty quick. I'm 18 weeks and tonight I struggled. It took me a good 10 minutes, I eventually found it (it wasn't very loud either) but I was surprised how long it took. I didn't even hear the normal swishing from the fluid and stuff. Should I be concerned? I just ate idk if that had something to do with it or maybe he was just deeper and resting idk. Any opinions to ease my mind are greatly appreciated 🙃