I was having problems last year and thought it was resistance but it turned out to be a lot simpler. I'd been injecting at the sides of my stomach, thought I was maybe just putting on a bit of middle aged waist fat, but turned out to be a small lump of lipohypertrophy which messes up absorption, so moved injection sites away from there to stomach proper. Also started paying more attention to pre-bolusing to about 20 mins before meals. These two changes have made a big difference.