if possible, try to take a picture or write down the part number or more likely a color code from your rear springs. They are most likely faded, but you can try. This may help or may not to identify what the culprit is and possibly to give you an advice of which springs to get instead.

I may try to find that by your VIN, so please give it to me, but chances are very low.
In general it is not easy with color codes / part numbers when buying springs. Your shock may have a different point (heighwise) of spring mounting plate.

The reasons for low back end:
Shock is tired as a good shock (especially gas pressure type) will have SOME lifting power.
Springs are tired if they are old.
Wrong shock-spring setup
Rear bearing rings are tired (for example 441512331 to know what i am talking about, yours may or may not be different)

You mentioned changing shocks and springs. Was it brand new set or used second-hand.

I do have a very similar problem in my C3 200

Good luck and don't forget to keep us updated