I use a Polar heartrate monitor myself - it helps to make the contacts wet (depends on the system I guess - I just let water run over my strap and the contacts of the actual monitor).
If the strap is very loose this could be a problem too. Could you just cut the strap and sew it back together in a tighter position?
Is it possible that the band of your bra interferes with the monitor (some of my sportsbras tend to flip the monitor over)?