Transmission problem?

I have a 2005 Nissan Altima. Recently, I encountered an Automatic transmission problem which is when I tried to park my car, the gear box stick will not go into P. and then I found out if I get out and push my car a little bit, it becomes fine. The transmission is working fine with the driving portion. It happened many times this week. I guess it must be something wrong with Gear stick. Any advice? Thank you.