Good Evening Folks,
My husband and I are attempting our first time floor installation and we have run into a problem that we cannot figure out and looking for some guidance.
We are installing 2 1/4in. hardwood flooring 3/4 in. thick and using a Pneumatic nailer with 2" staples and every time we staple in a section it tilts the board down so far we have to end up shimming every board to bring it back up. We have tried to decrease the pressure on the compressor by 5 lbs. at a time and then the staple does not go in all the way. We can't seem to find any other suggestions on how to correct this issue

Are you standing on the board that you are stapling? The groove of the board you are stapling should be over the tongue of the previous board. That and your body weight should not allow the board to tip. The only board that wouldn't have it's groove held by a tongue would be the first board in the room and this board should first be face nailed.
The tool is a flooring stapler that you hit with a mallet....I assume?

This may be a mis-mill problem. Does the side matching look OK? The boards must have the correct side match and you need to check by sliding boards together loosely without fasteners. Staples do pull the flooring down securely, but the next board should slide together without any problem, even if the sub-floor is only one-half inch thickness, to keep things in alignment. The shape of the edges of the boards should allow easily slipping the next board into place. The stapler should have 80 psi minimum for it to work well. Using a wax paper like AquaBar can help sliding into position, besides keeping the area from getting as dusty from all the banging and puffs of air from the stapling of a floor.