Assume that the economy is already in a recession, and both the President and Congress have decided to do some

Question

Assume that the economy is already in a recession, and both the President and Congress have decided to do something to restore the economy. Both agree that lowering taxes would not be a good idea, but do believe that it is in the best interest of the economy to increase government spending in defense, education & infrastructure.
The President and Congress change the budget accordingly, but after 18 months, GDP only increased by three quarters of the expected amount. What factors might be responsible for this situation?

Answer 1

if increased spending was financed by increased taxes, it would have no effect.

Answer 2

They probably didn't take into account that the increased income from government spending was being taxed, so they would have to reduce taxes slightly to increase other spending.