Hi Alison,
The only scenarios where layers should disappear from the legend are:
- Removing the layer from the map
- Checking the Legend item "off" in the Contents Pane.
- The layer is not visible because it is turned off in the map, or it is not visible because a visibility range has been set on the layer and the current scale on the map frame (that the legend points to) is out of this range AND the legend property "Layer visibility" is checked on (Format Legend > Options tab - Synchronize with map group).
- The legend item (layer) has "Only show features visible in the map extent" (Format Legend Item - Feature Display Options group) is checked and the map extent is such that no features of the layer are visible. This is set for individual legend items, which requires a layer already present in the map, so I'm not sure how you are able to set this and THEN add the layer. Perhaps I'm missing something?
I'm not aware of any outstanding issue where legend items unexpectedly disappear from the legend. Unfortunately, if you are experiencing a bug we would need more information to investigate further.
You mentioned that you are doing some copying and pasting. If you are copying and pasting layer elements (such as map frames and legends) make sure that the pasted element is referring to the correct map ( in the case of a pasted map frame) or map frame (in the case of a pasted legend). You can find this info in the Options tab of the properties pane (Format Map Frame & Format Legend).
Hope this helps.
Tom