A sore or uncomfortable site is a sure sign that insulin will not go in properly.. And you should change it straight away.
I can feel mine not going to be good just by pricking my skin with the needle nowadays. If i feel the prick of the cannula needle on my skin then I move it to a slightly different place.
Confession: I leave 2 sets removed (but glue still visible) on my skin to make sure I don't use the same place by accident. On my 3rd set on my left cheek say, I will wipe off the glue. Because the next set will go into my right bum cheek and vice versa.
I only use my bum cheeks and have done so for 30+ years only using my legs if in public restaurants for injections. Can't wear sets in legs or stomach as too slim and muscly.