[dmitril]

There is no backwards for a spring, newtons 3rd law. Also generally springs do not degrade from being stored compressed, rather repeated compression and more likely over compression will kill it. Also from my experience airsoft springs of lower quality tend to be over compressed by aegs hence loss of performance, but this will still take tens of thousands of rounds. Most likely your hop rubber is bad assuming you fixed compression. That was the case for my friends jg g36.

Edit: missed the fact you got it used in which case entirely possible for the spring to be worn out.