The times I've had crazy fast drops from a bolus have mostly been when the needle went too deep and made it act more like intramuscular than subcutaneous. In the early days(I was diagnosed in my teens so always self managed) I used more injection sites, but have found that the only ones that work predictably for me are the abdomen and the sides of my butt. Something about the consistency of the fat there. Trying shoulders, arms or further round or down the thighs I would get big variations in speed from one injection to the next, even with 4mm needles.