I could not feel my string either (and neither could any of my doctors I had over the years), and I had my IUD for 9 1/2 years. The first time I noticed no string, We had a ultrasound done, and My IUD is in it's proper place, so I left it there. Now I was spotting almost everyday, and I went back to the doctor to figure out why. He thinks my string was irritating it. I think my last pap smear trigger the spotting/string irritation. My doctor thought he would have to do a surgery to remove it, because he had trouble finding it as well. BUT he did found it, and removed it safely and I haven't spotted since.