Guys,
I have a MS Swift, just a 4 months old and I also have the reverse gear problem. I figured out what the issue is- lack of syncroniser for reverse gear. I also have problems with it not going into gear in 3rd very often with that grinding sound.
The problem may be common to the I20 as well because many of these manufacturers source parts from the same OEM.
To get around this problem
For reverse, make sure car is very still while engaging it using brake.
Then put it into 1st and then into reverse without releasing clutch. The 1st gear has a syncroniser, so from there if you put reverse, it will go into reverse. That's why that technique works.
For the forward gears, you must be sure you engage gear exactly correctly in that gear. In my case, the gear doesn't go in when I am lazy in changing gear and don't ensure I have the stick in the right place before releasing clutch. As I sit a bit back while driving, the gear stick sometimes doesn't move all the forward when putting gear. Doesn't seem to happen in 1st but when its time for third, I am usually negotiating traffic. When going in 5th, I am usually cruising, so doesn't happen there. Only happens in third.
Ameyam