Mine varies but I do have a real wriggler in there. My doppler arrived at 15 weeks and I found the heartbeat very low down above pubic bone on the right hand side.
Now at nearly 17 weeks little one likes to play hide and seek so I'll find it low down, then baby will think 'no, I don't like being poked' and move up to just behind my belly button. Once there it will let me listen for a bit, then disappear off swimming again.
The key is very small slight movements and try different angles as well as different locations.