Do you have an infant seat that you’re pulling out of storage for another child? If so, flip the bucket over and check out the harness. Some infant seats (and convertibles too!) have 2 harness lengths from which to choose—a newborn setting and a setting for larger infants/toddlers. If you’re like me, as soon as your child is done with the seat, you promptly stick it in the closet and forget about it without adjusting the straps; but, that means that that when you’re ready to use it again for a newborn, it’s set up for a larger child.
The following picture is from a Graco SnugRide manual (but it’s generic enough to work for other carseats) and it shows the 2 different loops where you can attach the harness to the metal splitter plate on the bottom of the carseat. Working on one side at a time, take the harness off the splitter plate and reattach it using the inside loop to shorten the harness. If you have one of the SnugRide models with a higher harness weight (this doesn’t apply to the SR with a 22 lbs. weight limit), you’ll also be able to adjust the harness length at where the leg straps are attached at the back of the seat. Doing this will mean you’ll be able to tighten the harness properly on a noob.