Hmm, can be tricky to get a heartbeat at 15 weeks. Try quite low down, just above the pubic bone. You might need to angle it slightly but move it slowly, not too quickly or you'll miss it. Use loads of gel. Baby's heartbeat sounds like a train, or galloping horse whereas yours will be a thud, and the placenta will be swishy like the sea going in and out.
Don't panic if you can't find it, many midwives won't even try until 16-18 weeks as it can be so hard to hear.
Don't use ordinary moisturiser, use the gel that is designed for it, makes it easier. My lovely GP is happy for me to 'pop in' for a listen and she does it immediately (of course) so you could try that if you want reassurance? (I am aware that I might be v lucky wit my lovely GP!!)
You need to use some kind of gel, and the best thing to check the heartbeat is early in the morning on full bladder. I started checking my baby's as early as from 12 weeks, so as long as you know what you are doing , you will find it. Whenever i tried to do it after I empty my bladder, I just would not find it. So full bladder in the morning is the way to go good luck next time