Energy Usage, Conservation and Weather Stripping - Attic Insulation

I noticed this winter that my 2nd floor was about 4 degrees colder than my first floor. Also, my single floor attached family room was also about 4 degrees colder than the rest of the 1st floor. This is something that I think has progressed gradually over the years.

Something they have in common is that they're both under a roof. My house was built in 1986. The attic has blown-in insulation. I've been told that as this insulation settles it loses some of its insulating properties.

Could that be the reason for the difference? Would it make sense for me to add insulation? If so, how should I determine how much to add?

I am sure it would help, but some other people on here will chime and help more than I will. You may have some air leaks and what not, so look for where air may be entering and seal that before you blow in more. But I am a huge cellulose fan. It does settle after time, so blowing in another 6-12 inches will surely help! If you do blow in more, make sure you don't block any soffit vents. You will need to buy some rafter vents to help prevent this.
You can rent the machine right from lowes or hd. It is super easy to run, but you will need a friend to help you out, moving the machine around and loading the hopper!

Effective insulation is always based on trapping air, stopping it from moving and the most effective insulation is polystyrene foam, foam is made by trapping small ball shapes of air and surrounding them with a very fine layer of plastic. If an air gap is less than 16mm the air does not move heat across the space by convection and the still air acts as an effective insulation.
The wind does not blow through closed cell insulation and strip its heat,nor does the wind reduce its effectiveness when blowing over it, nor does the heat from downstairs rise through it like other forms of insulation.
Nor does water vapour transit. This last is an important issue as things that are damp or wet channel heat out of the home at a much higher rate than dry things.

Polystyrene is therefore more effective than cellulose, fibre glass or most other affordable insulation's, it is also better in as much as, you can stand things on it without it compressing or deforming or loosing its insulation benefits.

Carefully fitting polystyrene sheet between the joists or better still spraying in foam will give you a warmer home for the same heating cost.