I find my back doesn't work ( I tried in desperation once and it was like the insulin wasn't working). The above knicker strip works for me, but even the , I occasionally get an area that doesn't seem to absorb properly. I then move to another place on my bottom.
I'm not familiar with the sets you're using, but I use a different set for my bum than my thighs. I either use a longer Teflon cannula or a stainless steel needle. I think this is because the bum area is kind of fatter and also more 'solid'.
I've not noticed problems with boluses. I tend to bolus in little amounts anyway and often bolus separately for main meal and dessert. Could you also try altering the speed of the insulin delivery on your pump?
As a last resort, you could use a pen for the bolus (in your leg or tummy) When I firstbswapped to my bum, I got so tired sorting out the initial highs. I bolused with a pen to give myself a break from the highs. That worked well.
I do,find my bum is less sensitive/absorbing.
Hope you sort it. I know how frustrating highs are.