NX=X-M , Therefore, only changes in X & M curves will shift the NX Curve.For example, government promotion on export may increase the export and shift the nx outward. While decreasing prefernce for imported (e.g. defence for foreign goods), may shift the nx upward too. Changes in income may only cause movement along the curves , while changes in factors other than income will shift the curves.

Fiscal policy may affect income by changing T or G. but it does not affect X & M Curve. Thus, with increasing income , the economy will move along the M curve only.

I dont know wheather I solve your question, anyway I have tried my best. Thanks