This usually comes from being nervous about fixing the right amount to begin with, in fear of doing too much. You need to experiment and find your perfect dose for a single shot, the one where you get the best rush without going over board. For some it's one bag, for others like me (depending if it was good dope) it would be 3 or 4.
With me personally, I could keep shooting up within 10-20 minutes of an initial injection and get the same rush, if not better if I upped my dosage.
Having a reliable source really helps with this because you already know the quality and can therefore figure out your perfect shot easily. If you cop different shit all the time, it will be hard to know how much to do without buying a decent amount and taking a trial shot of a bag or two to see how hard it hits you.