Does The Use Of Coumadin Have An Effect On Blood Clotting?

My brother in law is on coumadin and is in the hospital because they can't get the blood clotting measurements in the right zone. It's been 1.7, 1.5, now 1.7 again. What could cause this? He's taking some other medicines, but drs have reassured him that they are not interacting with the coumadin.