Timing Chain Installation Question

A properly made Woodruff just plain fits. But there may be a burr in either one of the keyways, or dirt,

OR the key itself may be just a little too tall, OR you may have gotten the wrong key.

I agree with the above, "trial fit" the thing. You may have to file some off the top or bottom to get it to seat. SHOULD NOT take much, just a stroke or three

Another thing you might try is to find some soft material to protect the cam, even a folded up section of soda can material, then squeeze the key down into the keyway with a pair of slip-joint pliers.