Ah, well if these windows are 15 years old then I would wager that the balances are just a little too old. Most channel balances have a lifespan of 10-15 years, depending on how often they're used. Once the springs begin to wear out, they can get stiff and you'll find yourself struggling to operate the window.
Before you consider replacing them, though, I have a few tricks you can try to give them a few more years of life. First, remove the sashes and balances from the window entirely and look over the tracks where they install. Make sure they're nice and clean with no dirt or debris getting in the way. Then, once you're sure that the tracks are clean, coat them with a silicone spray to help lubricate their operation. We like silicone over WD-40 because it doesn't attract dirt. More often than not, WD-40 will just make matters worse and gunk up the works.
If you're still struggling with them, you can apply white grease directly to the balance springs. That might help, as well.
If all this fails, then I think it's about time you consider new balances for these windows.