Make sure your name & DOB on your reservation match exactly what was on the Global Entry/NEXUS application form. (Go to the GOES website and click on your NEXUS/GE membership...it'll let you see your original application)

The APIS system which actually handles this is quite dumb and inflexible. If you put a middle initial for your middle name in one place and have it fully spelled out in another, the system will say you're not a match to the KTN.