I have a heart monitor from previous pregnancy and thought I would give it a go. For the last 4 evenings I have pocked up heart eat straight away but only 1 each time, I can't seam to find the other one.
Home-use dopplers are honestly more trouble and worry than they're worth. Unless you're a trained medical professional, it's really hard to reliably find a fetal heartbeat, even at the later stages of pregnancy. It's also very easy to find a 'heartbeat' through the placenta, which has led to some terribly sad stories of mothers who were concerned about lack of fetal movement checking themselves out with a home-use doppler, finding what they thought was a heartbeat and then going on to lose the baby.
You should think of your doppler as a bit of fun rather than a piece of medical equipment. If you are concerned about your pregnancy in any way, call your midwife or other HCP straightaway and get yourself checked out.
I took my friend who was 34 weeks PG with twins in for her birth and waiting with her until her husband arrived. While I was there, they hooked her up to a doppler. They had no trouble finding one heartbeat but terrible trouble finding the other and even when they did manage to find it, they would lose it after a few seconds. This happened repeatedly with a trained medical professional and hospital equipment. So if the professionals have trouble so late in PG, it's no wonder you are! If you are worried for other reasons, have a chat with your midwife.