Keep trying, I think it took me till week 11 to find it properly

Use the gel if you have it, if yu don't have any, try the durex play range of lubrication, hehe - dual purpose.
Go really low down, just above the pubic bone. And don't panick if you can't find it one day, the baby moves around so much it may have gone further in.
You can pick up a lot of heartbeats which are your own, the baby's is much much faster.
Hope that helps.