It could be one of three things:
1. your monitor doesn't come on fast enough.
2. it is displaying the startup screen on a different video interface. (For example, you have a DisplayPort monitor and the video card insists on displaying the bios POST on the DVI outputs only.)
3. your bios is set to UEFI fast boot, which reduces POST to the bare minimum
There's a fairly easy way to get into the bios under said circumstances: as soon as you power up the machine, tap the delete key once per second until you enter the bios. I have to do this all the time.
Wanna know what's worse? Older IBM AIX servers. I have a couple AIX servers that run through a 20 minute diagnostic when they first power on, then give a 5 second window to hit the 5 key to tell it to boot from a CD or DVD. Imagine having to sit around a machine for several minutes to watch carefully for that brief window to hit the right key. It's so moronic. Those IBM engineers need to have their heads examined.