Any problems getting the old plugs out? Sometimes they just don't want to come out seems like. Put anything on the new plug's threads?
I changed the plugs on my 2010 5.3L back in September, it had about 135,000 miles. not problem getting the plug to break loose, only issue was accessibility on the passenger side rear 2, got a little tight, but I am a mechanic and was doing it on a lift and found going from under the engine was easier. I didn't put anything on the threads when I went in either.
only time I have seen spark plugs stick is in Ford engines, or with people who wash their engine regularly and the spark plug oxidizes to the head. you know, steel spark plug, aluminum head, different metals having electricity travel through them causes problems when they have water added regularly.