Hi,
I had the same problem quite frequently before. If you don't vary the insertion site then you will end up getting scar tissue which will prevent the insulin from being well absorbed. Another way of getting bad scar tissue is if you don't change the site every 3-4 days (I used to keep it for more like 7 days as I didn't know this). It's quite a dangerous thing as I tend to just keep bolusing thinking I ate too many carbs when in fact the insulin is just being absorbed very slowly...
Oh and yes, when I was getting this I also changed from 6mm to 9mm just to be sure.
Obviously there's always the possibility of it being kinked. My philosophy is that if in doubt just change the set...
Hope this helps a bit. Take care