It is extremely important that the correct Staff is active when you insert a symbol (if you are editing multiple staves, of course).
If you for example insert a symbol with the wrong staff active it might later appear like the symbol has disappeared simply because you edit another configuration of Tracks (the Track you actually inserted the Symbol to might not have been opened for editing).
The same is true for Note Symbols and their relation to Voices. Make sure the correct voice is active when inserting symbols or they might wind up at the wrong position, fermats may be turned upside down etc.
Layout Symbols work slightly differently. Instead of belonging to a certain Staff or Voice, they belong to a Layout. Since different Track combinations use different Layouts, this means that if you insert a Layout Symbol in the score when you are editing two Tracks (for example a trumpet and a saxophone part), it will not be there when you view each Track by itself in Score Edit. However, you can copy Layouts between Track combinations.