home fetal monitor

So I'm 17 weeks.  I was at the motherhood store this past weekend and bought a Gerber home fetal monitor ($25)....well, I can't hear anything!  when I read the directions it says that it works during 3rd trimester!  ugh!  it didnt say anything like that on the box or I wouldnt have bought it. 
So has anyone had any experience with the gerber monitor?  or is there one that will work now that I should be looking for?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370284369 810&ssPageName=STRK:MEWNX:IT

I have this one - it was $50 and I've been hearing the baby's heartbeat since 12 weeks - LOVE this thing! So sorry that the one you got isn't working well - I'd return it!

thanks for the info....i'm going to give that a try....and will be returning the other one.