2005 Nissan Sentra Bought Car Weeks Back Initially Driving Car Nice

I bought the car few weeks back. Initially, when I was driving the car, it was all nice. Suddenly, after 1 week or so, I started feeling that the gear change from P to R, and R to P is hard. Not as easy as for other changes. I talked with the dealer from I bought the car. The guy showed the car to a mechanic and the mechanic opened the box and cleaned. Then it became little better, however still not smooth as expected. what could be the problem and what should be done ?

Thanks in advanced