Be sure the notch on the clip is facing upwards and outwards from the Castrator. Check to see if the clip is damaged or bent. Sometimes burrs develop in the slot the clips slide into. Take a knife and shave these burrs off or until the clip fits into the slot. Also, if you are using the “Original” EZE Castrator, the newer designed clips are round whereas on the original Castrator, the slot that holds the clip is oval. The newer clips will still work if you take a knife and shave the lower portion of the hole or simply bending the clips slightly to an oval shape. Sometimes, the clip may fit snug in the slot also. Just push on it a little and it should go in. You may also place the clip on your pinky finger of your hand and place in the slot. This also may help sometimes.