I would say wire also.
When I changed my plugs, almost half of them seemed glued on, never seen clips in the boot so tight.
It is hard sometimes especially when getting frustrated, not to grab onto the wire to help pull, but if pulling on wire instead of the boot, the wire can pull out of the socket that clips on the tip of the spark plug.

You could take each one off and inspect and test ohm to see if connection is good.
Or just buy another set.