When a plant stops growing, it can be for any number of reasons. Sometimes it's due to insufficient water, light, or fertilizer. Sometimes it's due to an overabundance of those things. In other cases, it's due to a lack of stimulus--nothing's changing, so why should the plant?

In this last case, it may be possible to shock the plant into new growth by cutting back some of the older growth. This is a delicate operation, though; cutting back too hard will result in the plant's slow, lingering death . . .