If they disappear after you place them, check your sheet setup. If you haven't configured your symbols to go to a spesific sheet, they will go to the current one, which may very well cause them to end up below the floor or other obstruction.
In the symbol options (accessible from the symbol manager) you can enable the Force Sheet option, and set the appropriate sheet (Usually SYMBOLS*). Note that you usually want to do this on the symbols in the actual symbol catalog, and not just the symbols in the current map, but also note, that since symbol definition is copied from the symbol catalog to the map upon first use of any individual symbol, if you change a symbol in the symbol catalog, it won't affect those symbols already used in the current map.
If you don't want to mess too much around with this, you can also just remember to have the correct sheet active when placing them. Symbols go to the current sheet if nothing else is specified.
Comments
Note that you usually want to do this on the symbols in the actual symbol catalog, and not just the symbols in the current map, but also note, that since symbol definition is copied from the symbol catalog to the map upon first use of any individual symbol, if you change a symbol in the symbol catalog, it won't affect those symbols already used in the current map.
If you don't want to mess too much around with this, you can also just remember to have the correct sheet active when placing them. Symbols go to the current sheet if nothing else is specified.