Why is my Canon TS8150 not printing some items of text

Why is my Canon TS8150 not printing some items of text

My Canon TS8150 printer seems to print ok, except that it fails to print some items of text. When I print a test page it just prints the blue header but not the rest of the text, and if I create a Word document it misses out some of the heading text. I suspect the firmware in the printer is corrupted, is there any way of reinstalling the firmware? If I try a firmware update it fails because it says the printer is already up to date.