2002 ford focus 2.0L ztec shifter problems

Hi there I have an 02 focus wagon 2.0L Ztec I have a problem with the shifter in it when i go to put into drive or reverse some time it will go but sometime i have to use a flat head screwdriver and push the little button beside the shifter to get it to go into gear i am just wondering what would cause this any help would be greatly apperciated