I have a few different types of gamepads, please describe which one you are referring to and I will do my best...
I have the ADAPTOIDs with 3 N64 controllers (single analog, 1 digital).
Also, I have 2 of the lik-sang.com PS2 Dual-shock / N64 controllers with 2 PS2 dual-shock (dual analog, 1 digital).
And 3 of those Gravis Gamepads (2 Gravis pro ps-1 style, 1 gravis snes style)
FIRST, make sure you have calibrated the Controller under the Control Panel in Windows.
Now, as far as sensitivity on analog pads, it's kinda difficult to see what you're talking about. In most of the controller plug-ins, there's different settings for Analog dead-zones and controller sensitivity (N-Rage`s comes to mind).
For DIGITAL pad sensitivity, there's also some issues. If you push the button and the game does not respond, that's something on your end. But, perhaps on digital pads there is a keypress repeat or something, which is some type of setting somewhere which will tell the computer how many times per second to record holding down a button.
Don't really have problems here, try using N-Rage`s plugin to get the more robust features, or spend $5 at lik-sang.com for the generic N64 adapter.