Yeah, it sounds like the spring strength might be mismatched. What is the weight of your sash and what option did you select when you purchased your balance?
I bought the 2140. My window was right around 15 lbs.
Hmm... 2140 should be able to support that weight, though I admit that 15 pounds is right on the cusp.
You mentioned that the original stamp is C21. That may be something like a 21C stamp, which corresponds to our 2130 option. Weighing the sash itself is usually better than relying on the stamp, but in this case I think maybe going down to the original 2130 spring strength is your best bet. Also triple check your sash weight just to make sure its not a less than 15 pounds.