It sounds like you edited the layout itself, creating a page override (little red triangle on the page icon on the top right). If so, master page changes will not be applied to that page. If that’s the case, you need to right-click on that page icon and “Remove page override.”
If you added the Text as text in Engrave mode because using the File > Project Info resulted in text not appearing or being cut off, you need to enlarge the various Text Frames and add the appropriate tokens–on the appropriate Master Page(s)–to produce what you want. Text Frames do not expand to hold added lines (or large sizes) of text. You have to resize them yourself.
If you’ve made edits to individual pages and then later change the master page, the individual pages aren’t updated - this is because manual changes to individual pages are overrides, and Dorico therefore preserves the state of the page as you’ve left it.
Centering text - this came up earlier today in another thread: a good way to center text is to add text frames that span the full width of the page, then center the text’s alignment. You also don’t have to have a separate text frame for each token: you can have multiple tokens and multiple lines of text in a text frame, just make sure (as Derrek says above) it’s tall/wide enough to accommodate the text. Separate lines of text can have different paragraph styles - so you could have the dedication in the same frame as the title, but apply a separate paragraph style to it to get the correct formatting (italic, smaller font size)
