A full harness should be used to about 40 lbs. Find and read the instruction booklet. Look for a convertible seat that has a higher weight limit in the rear-facing position if your baby is less than 1 year and over 20 lbs. Try locking and releasing the buckle in the store. Try changing the length of the straps.
Some harnesses adjust automatically to fit the child.
Many can be adjusted easily from the front or the side. Others have a metal adjustment slide through which you must pull the straps. This keeps the strap from pulling through the slide in a crash, which would allow the child to be thrown out. Try the seat in your vehicle. Most safety seats fit better into some vehicles than others.