My guess is those insturments *are* in the score but are marked invivislbe in Edit / Instruments/ Or, you are being confused by the fact that a handful of instruments (eg, trumpet, violin) show multiple channels for the different sounds those instruments can make (trumpet open versus muted, violins arco versus pizzicato, etc).
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Welcome, attach here your score and provide some more details.
Also try https://musescore.org/en/handbook/revert-factory-settings
My guess is those insturments *are* in the score but are marked invivislbe in Edit / Instruments/ Or, you are being confused by the fact that a handful of instruments (eg, trumpet, violin) show multiple channels for the different sounds those instruments can make (trumpet open versus muted, violins arco versus pizzicato, etc).