Play with spring legnth, as well as spring rate (stiffer spring). Two different things! I have used a torch and re-bent a spring into a two stage/dual rate style. I have a little experience bending wire and heat for orthodontic wire. Same basic thing. Or try shims/washers to add a little more force behind the spring. Sewing-machine oil is excellent!! I have tried all types of oils and grease.
Don't be scared to reach out into other applications to track down a spring. Could be, that the perfect spring came from a barn door, or a inside a stuffed animal??