Check the cosmetic condition of the midi keyboard. Is there any evidence of damage or spills on the keyboard that could have contributed to the keys being stiff? If you notice any stains from a liquid being spilled into the keyboard, you will need to clean many of the components inside the keyboard.
Play the keyboard. Play each key running scales up and down. The more you play the keyboard, the more it will begin to "break in" and whatever gunk is inside may wear itself off. If it is a brand new keyboard, the only way to break it in is hours and hours of play time.
Remove the keyboard's plastic panels and examine the keyboard on the inside. The contacts may be covered with dirt, hair and other debris that will stiffen up the keys. Remove all dirt and dust that you see with cotton make-up pads and rubbing alcohol. Dirt and debris in a keyboard can damage its components and lead to "dead" keys, or keys that don't produce any sound.
Turn all of the pots back and forth and slide the sliders up and down. If your playing requires you to make quick adjustments with the pots and sliders, you will want them to be relatively sensitive and not hard to turn or slide. Break them in by repeatedly moving them around; they will begin to move much more easily the more time you spend using them.