Not nearly enough information to make a deduction. At this point, about the only thing that can be said with any degree of certainty is it's an issue related to root health/function.

Does your pot have a drain hole? Does it have an attached collection saucer? Do you allow water that exits the drain hole to be reabsorbed by the plant? What do you use for fertilizer (exact product including NPK %s)? How do you determine when it's time to water? When you water, do you provide just enough to moisten the soil, or does a good deal of water exit the drain hole? Does your home have an ionic exchange water softener (uses salt as the exchange medium)?

Al