acoustic flat top string playing action is predicated on these basic criteria ...
nut relief (tweaked via the the nut slot depth)
neck relief (tweaked via the truss rod)
saddle height (tweaked via sanding the saddle bottom to lower or shimming to raise)
neck angle - this is critical as changing this angle typically requires a very invasive and costly neck reset. neck angle can be verified with a straight edge placed down the center of the neck and observing where the lower corner of the straight edge meets the top of the bridge (not the saddle) - if the straight edge meets the corner of the bridge perfectly, or is slight above (about 1/16"), the lowest possible action can be achieved ... if below the bridge corner, even a little, lowest string action will not be achieved.
"top hump" - all acoustic flat tops will eventually hump up behind the bridge, causing the bridge's rear to rise, which causes the string action to increase. adjusting the truss rod and/or lowering the saddle are the quick remedies. for severe cases, a jld bridge doctor installation will pull down that top hump and improve the tone in the process.