One thing I do if I have to replace a dodgy set is when I take it out I then reattach the reservoir with a bit of insulin or even water in and screw the plunger back in, then try pushing liquid through first just the tubing and then with the used cannula reattached, it gives me an idea if it was likely to be a blockage in the cannula itself or if it was in the tubing, or in fact if there was no blockage at all. It doesn't solve the problem as by the the set is out and replaced but when I was having a lot of problems it did help narrow down there the problem was happening which helped me find a better way.