... what you are encountering is the various zones in which items are permitted to be displayed. For example, if you set the staff height too short or place the staff too high in its range, you will not be able to place dynamics or tempos very high above the staff, because there is no room. ...
... I notice also that dynamics placed on the score in continuous mode can act weirdly if the measure to which they belong happens to be the first in a new system: sometimes the selection box appears at the end of the previous system, even if the mark itself appears in the proper location.
I frequently encounter the problem of "dis-associated dynamics" in three contexts:
1 When Page Set-up (or inserting bars) has moved a note that is associated with a dynamic to a new line (picture)