@AuntieM
I think you may be "Screen Mirroring" rather than "Casting". Similar, but a little different.
Look for something on your phone that "enlarges" the screen. May be a big box icon or two diagonal arrows. (usually located near the video/media controls.)
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Post some information with the app you are using or maybe provide a screenshot and see if we can help a bit more with some more details. To post a screenshot here, upload using the little camera icon in the message reply box.
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