Window Glider/balancer in Frame of Double Hung tilt NuSash window

It definitely sounds like you need a new coil balance assembly. I can't quite make out what you have from these photos, but your pivot shoe is similar to our 15-199. Look this part over and see if you agree.

If so, then the 15-199 is compatible with two different springs: the 3/8" Series 165 and the 1/2" Series 160. You will have to measure your original spring to verify which of these two, if either, is a  match.

If neither matches up, or if you aren't sure, then please show me a phot of the complete balance assembly out of the window and without obstruction.