Hi rick,
the plugwire cap should snap onto the plug with a minimal effort push.
If you are installing new plugs, check the plug's end and the cap's innards.
If the plug has a smooth end and the plug cap is designed to snap onto a threaded end it won't go on.
Most smooth plug ends will screw off to leave a threaded end.
If not, get plugs that have that feature or get plugwire caps that are meant to snap onto smooth-ended plugs.