Problem adding files to data dvd


Having written a data DVD (with Nero and trying to add files later on, I come across the following:

The DVD writer (NEC 3550AG) is able to read the newly written data but not the previous.

I can access my previous data from my other DVD (Acer) but cannot see the new ones.

1.Am I doing something wrong?
2.Is there any way to add data to an already recorded (not finalised) dvd and be able to see all the data on the same driver?
3.Can I also direct the new data in already existing folders? I never had problems doing that in the past with data CDs.

Thanks in advance.