You are dragging the fill handle to include cells that are read-only.
You are dragging the fill handle to include rows that do not have any values in one or more columns that are marked required.
You cannot fill values in a calculated column by dragging the fill handle. A calculated column gets its values by evaluating a formula. To view or edit the formula of a calculated column, click a cell in the column, then click again.
You are attempting to fill cells with values that would fail the cell or row validation rules.
You are attempting to fill cells with values that are longer than the maximum length allowed in the destination cells.