Excel auto calculation formula question.

I'm using the drag function to drag a formula from horizontally.

My formula is simple =SUM('Enrollment Data'!H15:L15)

When I drag it horzontally I want it to go like this.
=SUM('Enrollment Data'!H15:L15)
=SUM('Enrollment Data'!H16:L16)
=SUM('Enrollment Data'!H17:L17)
=SUM('Enrollment Data'!H18:L18)

Instead it's going like this
=SUM('Enrollment Data'!H15:L15)
=SUM('Enrollment Data'!I15:M15)
=SUM('Enrollment Data'!J15:N15)
=SUM('Enrollment Data'!K15:O15)

I tried using $ infront of the letters but it kept the vale all the way through. Can anyone help?

Thx.