When was the last time you repotted? If it's been in the same pot for years it might have gone through a lot of the good stuff in the soil already, if it hasn't been washed out through watering.
Have you looked I to fertilizers? Might help avoid fully repotting.
I'm in no way a professional at this stuff but I ran into similar issues with some pepper plants I've had for years and they definitely perked up after repotting.