Undo Only Going Back About 9 or 10 Steps

I'm sure there are better uses to put Excel to, but I have a Sudoku Solver which helps you see if you've gone wrong at a certain point, and then you must undo your 'moves' (entries) back to when the cell-pointer lands on a particular coloured cell. This could require 15 or 20 back-trackings (Ctrl-Z). Could be my fairly new laptop, but I found the Undo was stopping after only about 9 or 10 backward steps. It should be a feature that can accommodate up to 100 steps backward, according to Excel 2016 Help. Has anyone else found their Undoing to be limited like this? Could be the laptop. It's Excel 365 that I've used on another laptop without the same problem occurring. Thanks.