This is a common issue with the Priv due to aging hardware buttons or timing. If you are struggling to get the timing right, try this "Boot Loop" method: how to open fastboot mode on blackberry priv hot
Before attempting to enter the mode, you must ensure the device is in the correct state.
BlackBerry implemented aggressive thermal throttling in the kernel (specifically in the msm8992 driver). When the battery temperature sensor hits 38°C:
To beat this, you must force the phone to think it is cold. There is a software trick if you are rooted (ironic because you need Fastboot to root): This is a common issue with the Priv
Without root, you rely purely on ambient cooling.
Before attempting any key combinations, you must cool the device. Do not skip this step.
Pro Tip: The Priv’s battery is non-removable, but you can place the phone on a cold, hard surface (like a granite countertop or a metal laptop cooling pad) to passively wick heat away. To beat this, you must force the phone to think it is cold
You will know you succeeded when you see the BlackBerry Priv Fastboot screen:
From here, even if the phone is warm, you are safe. Fastboot does not trigger the CPU governor, so heat will not kick you out.