A lot of people who have trouble with BDC, are having trouble positioning line item for access.
The solution is to find a repeatable pattern. Filling in row 1 then row 2 and so doesn't work because you run out of rows. Ideal would be to fill in the same row every time.
So, what I do is always fill in row 2 then reposition to that line number. Repositioning puts that line number in row 1 and then you can fill in row two again. Just put her into a loop until all rows are entered.
HTH,
Neal
PS: Avoid BDC when a BAPI exists as mentioned above!