There are a couple of possibilities. One is that the new rows are somehow shaded white, which will then cause the gridlines (and borders, if they are also white) to disappear. Select the worksheet, and go to Format | Cells | Pattern and ensure that "no color" is seleted. Also check the color of your borders.

Another possibility is that the gridlines were somehow turned off, and now are not showing when you insert new rows. Select the worksheet, and go to Tools | Options, and on the View tab, ensure that the Gridlines box is checked.

Also, check to ensure that when you are adding new rows, you are not adding it directly above/below a row that has cells joined. To check this, select the area that has no gridlines, and click Format | Cells| and on the Alignment tab, ensure that the Merge Cells box is NOT checked

I hope this helps. If not, let me know and I will do some further research.
Regards,

Barbara