The header rows for a simple tablix do not repeat on each page by default. Since I keep forgetting the solution, I posted the answer that I found on the internet here.
Posted by Stacia on http://stackoverflow.com
“It depends on the tablix structure you are using. In a table, for example, you do not have column groups, so Reporting Services does not recognize which textboxes are the column headers and setting RepeatColumnHeaders property to True doesn't work. Instead, you need to open Advanced Mode in the Groupings pane: click the arrow to the right of the Column Groups and select Advanced Mode. You'll see Static Groups appear in the Row Groups area. Clicking on a Static group highlights the corresponding textbox in the tablix. For the column headers that you want to repeat, select the Static group that highlights the leftmost column header. This is generally the first Static group listed. In the Properties window, set the RepeatOnNewPage property to True. Make sure that the KeepWithGroup property is set to After. The KeepWithGroup property specifies which group to which the static member needs to stick. If set to After then the static member sticks with the group af-ter, or below, it acting as a group header. If set to Before, then the static member sticks with the group before, or above it, acting as a group footer. If set to None, Reporting Services decides where to put the static member. Now when you view the report, the column headers repeat on each page of the tablix.”