If my IUD strings cannot be felt, should I see my physician?

Yes . Assuming you were able to feel the strings before, not feeling them now should be evaluated. Most of the time, inability to feel is simply because if their position in back of, or by the side of, the cervix. It also depends on their length (sometimes they were cut too short). Nonetheless, it's a good idea to simply have the iud's position assessed by your md by an exam or sonogram.