Can't safe boot in imac

Kurt Lang advises a process after moving fonts that includes a safe boot. Problem is, I've been unable to boot into safe mode, despite holding the shift key down and following the advice of articles on the internet.

Possible interference: my Mattias keyboard or having a Windows partition with Boot Camp.

Any ideas?

Do other Mac startup key combinations work at all? If so, that would make the keyboard less likely as cause.

The last paragraph on this page lists a way to boot in safe mode using Terminal commands, as a last resort.

Are you doing a warm boot (a.k.a. Reboot) or cold boot? The latter is what you want.
When you attempt the Safe Boot is the Matias keyboard connected via bluetooth or USB? (Third party bluetooth keyboards may not connect early enough in the boot process to trigger the boot althernatives).

Thanks. That Apple page got it.

For interested readers like me, with a Windows partition and a third-party keyboard plugged into a USB port, the procedure is this:

Open Terminal
Type the following: sudo nvram boot-args="x"
Restart or shut down the Mac
As it starts, hold option key, then choose your Mac OS partition
While it loads, immediately press and hold the shift key
After the desktop comes up, to verify that you are in Safe Mode, go to the Apple icon in the upper left corner of your screen.
Go to About this Mac, then System Report
Scroll down to Software
Next to software, it will say Boot Mode:
Should read Safe rather than Normal
To get out of safe mode, open Terminal again
Type the following: sudo nrvam boot-args="" (no space between the quotation marks)
Quit Terminal and restart the computer