What Does This Error Mean?

Nook font size not changing usually means the book has publisher-set formatting that overrides user display settings, or the font size slider in the reading options (Aa icon) was not applied. Try increasing or decreasing the font two steps and then tap outside the menu to apply the change.

Affected Models

Nook GlowLight 4
Nook GlowLight 4e
Nook GlowLight Plus 7.8
Nook Tablet 10.1
All Nook e-reader models

Common Causes

Publisher-fixed layout — some ebooks (comics, children's books, textbooks) have locked formatting that ignores font size settings
Font size change applied but tap not registered — slider requires a deliberate drag gesture
PDF file open — PDFs do not support reflowable text and cannot change font size in the same way
Nook app cache storing old display preferences over new settings
Touch calibration issue making the Aa icon hard to tap accurately

How to Fix It

Open the font size slider inside the book.

Tap the center of the screen to bring up reading controls, then tap the Aa (text settings) icon. Drag the Font Size slider left or right — you should see the text resize in the background as you drag. Tap outside the settings panel to close it and confirm the change is applied.
Check if the book uses a fixed layout.

Some ebooks, especially illustrated books, comics, textbooks, and children's titles, use fixed-layout formatting that cannot be reflowed. Open a standard text novel (not an illustrated title) and try changing the font size there. If font size changes work in a plain-text novel but not in a specific book, that book has publisher-locked formatting.
Check if you are reading a PDF file.

PDFs on Nook do not support font resizing the same way EPUB files do. In PDF view, you can pinch-to-zoom to enlarge the text, but the font size slider in Aa settings has no effect. If you need adjustable fonts, look for an EPUB version of the same title from the Nook store.
Restart the Nook.

Hold the power button for 3 seconds and select Restart. A restart clears the display cache and reloads the reading app fresh. After restarting, open the book again and try adjusting the font size — the slider should respond immediately.
Clear the Nook reading app cache.

Go to Nook Settings → Apps → Nook → Storage → Clear Cache. This does not delete your books or highlights — it only clears temporary display data. After clearing the cache, open a book and adjust font size again.

When to Call a Professional

Font size is a built-in Nook reading feature. If it consistently fails to apply on standard EPUB books after restarting the device, contact Barnes & Noble support.