So after a little review about ammonia, I think I need to rephrase what I said. Ammonia (NH3) actually is a base in certain conditions.
When pH is less than 7, ammonia (NH3) is able to pick up hydrogen ions from the water and become ammonium (NH4+). This effectively is a property of a base.
As the pH approaches 7, ammonium (NH4+) gradually loses that extra hydrogen and is then replaced by ammonia (NH3). When pH is greater than 7, ammonia (NH3) predominates.
So in acidic solutions (pH<7), ammonia has a slight ability to increase pH because it removes hydrogen ions from the water. But in basic solutions (pH>7), ammonia cannot take up the hydrogen ions, therefore it cannot increase pH. When we are talking about high pH conditions then, it is certainly not ammonia that contributes to it.