Crowding Out. An increase in government spending causes a decrease in private investment (maybe the government uses resources that private firms would have used).
An expansionary fiscal policy may have no effect on real GDP if government spending crowds-out private investment. An increase in government spending will increase money and credit demand, increase the interest rate and decrease investment. AD shifts right because government spending rises but AD shifts left because investment falls. If there is complete 100% crowding out then the increase in government spending is cancelled out by the decrease in investment so AD does not move and real GDP does not change.