From your explanation it seems that you've set the RowsCount property of the RadTileVIew to 1 and that's why you see the horizontal scroll bar. If you set the RowsCount to 0 (or remove the setter) the RadTileView will automatically place the items on a new row. You can also see the Rows and Columns
help article which explains how to change the arrangement of the TileViewItems.
Please if you have any further questions don't hesitate to ask them.
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