Kar,
If you can find the top of your fundal height (a hard ridge - should be just below your belly button at 15 weeks) then go down and find your pubic brim (hard ridge just above your hoohaa). Now, if I were you I would try to find your little one's hb just under the pubis (pelvic brim). Don't just put it on, push in and then angle it downwards, as if you are trying to get under the bone. Also, wait for a bit, often it takes a few moments for the doppler to pick up bubbies hb.
If no luck there, I would go to the top of your fundal height and place the doppler smack in the middle, remember to push down a little bit and angle it downwards.

HTH and GL