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Most printers keep an unprintable margin and shrink the page to fit inside it. In the app, turn on Print to the edge on the print screen. If the border is still there, your printer can't go borderless at that paper size — add Bleed in the same place, cut the paper slightly oversize, and trim after printing.
Printers don't agree on how they place an image on a custom paper size. On the print screen, step Turn the sheet through 90°, 180° and 270° until the preview matches what comes out. Use Print one test sheet so you only spend one piece of paper finding it.
Open the book and press Print again — already-printed sheets come up unticked, so you only send what's left. You can also untick any sheet you want to skip, or re-run a single sheet that jammed.
Ink on matte paper can't reach the colours a screen can. Raise Colour boost in Settings. Also make sure the Paper type in the app matches what's actually in the tray — telling a printer "photo" while plain paper is loaded floods the page with ink.
Pages point at photos in your library rather than copying them. If a photo was deleted from Photos, the page that used it will be blank. Put another photo in that slot.
Email support@myprintbook.com and tell us what printer and paper you're using — that's usually the answer.