The first time I tried mine, I did it dry and it still picked up just fine.
Maybe your transmitter is running low on juice? Or maybe there's a buildup from sweat on the strap? Not sure what brand you have (mine says its machine washable @40C/104F), but you can try washing it with a bit of soap to get any buildup off.
And like Roxie linked, there are gels you can use; I saw a bunch on amazon when I was looking for my HRM in the suggestion section between the product and reviews.
| current weight: 153.0