I had this exact problem, I removed the gear lever cover, I inspected how the whole gear lever things work. If you look closely, you can see that when you change any lever position that a long metal pole moves too. it has no obstruction from P to N, but when going into D, it has to fit inside a black plastic clip. I realized that the metal part was hitting the edge of the clip first, as I added pressure it would slip in properly. All I did was dremel that edge of the clip a little, and this made that metal pole slip in easy again.