[SOLVED] Rows Missing but not Hidden?

I was trying to auto-fill a column with numbers like

1
2
3
4

you know, simple drag and release. So I started off with

1
2

then dragged and ended up with

1
1
1
1
1
1

From what I can gather, having cleared contents in case of false blanks etc, the issue arises because the rows aren't sequential. Row numbers are as follows:

1
2
3
-
6
7
8
9
10
11
-
14
-
18

Where the " - "represent a thicker separator line than normal.
However there is no hidden line

Any suggestions on how to fix this so I can drag down the list? I have about 2,000 entries to give codes to.

Thanks