I am 28 weeks 5 days with my third child.

I don't know if I am being a worry wart. I have several apps on my phone that allow are suppose to allow you to hear your baby's heart beat. I haven't been able to find my baby boy's heart beat. I am really concerned. He is not that active in the womb but he has always been very calm unless I drink caffeine or soda. Can somebody give me some reassurance?