Depending on the way the flipper button switches (leaf only) are configured on your game, pressing the buttons very hard may actually result in a weaker flip, due to leaf blade bounce cutting the flipper power in tiny intervals. I've seen games with old/weak flippers to begin with, in which pressing the button softly will get a ball all the way up a ramp, while pressing it hard will not.

Somebody who has worked on actual Sterns can correct me if I'm wrong, but I assume Stern uses computer-controlled flippers, which means they are not directly connected to the switches. When a signal is received that the switch has closed, the computer turns on the particular driver transistor that controls the associated flipper(s). My guess is that they are programmed so a certain period of time must have elapsed with no switch connection before the computer will turn off the driver and thus lower the flipper. Presumably this is done to mitigate the effect of a not-quite-perfect switch contact. Not sure if this is adjustable or just the way it is on those games.

(Source: I built my own game with electronically-controlled flippers, and they worked exactly like that until I revised the software and hardware design.)