Gear shift sticking down when shifting down!

Hello. The problem started a couple of days ago on my 2000 sprint st. Sometimes when changing down from 3rd to 2nd the gear leaver stays down it does not flick back up but does ingage 2nd. I then have to shift back to 3rd and then go back down. This does not happen all the time and there is no pattern to when this happens. Does anyone have any idea of what is wrong. Cheers.