On the Utility Network Analyst toolbar in ArcMap, clicking Flow > Display Arrows causes the flow direction symbols to appear on some network features, but not all of them.
Flow direction symbols for edge features are drawn at the midpoint of the feature. If the midpoint of the feature is not within the current view, the symbol is not shown. Flow direction arrows are also not shown for features that may appear too short in the current extent of the map given the flow direction symbol used.
Pan and/or zoom the map appropriately until you see the flow direction symbols.