Cannot boot from USB sticks > 2GB

[panksnet]

I was unable to boot from most USB sticks I tried and would like to figure out why.
My computer only recognized as bootable the following USB media:
- a real USB DVD-ROM drive;
- an emulator of DVD-ROM drive (IODD2531 USB 3.0 External Hard Drive Enclosure);
- a few old 2GB USB flash drives I still owned.
The other flash drives I tested (4GB, 8GB, 16GB and even some 2GB) were not recognized. At that, all the mentioned USB sticks were recognized as bootable on my other computers. The HP Pro could use them too but only as usual storage  devices, not for booting.  

The BIOS settings I used:
Advanced | Secure Boot Configuration | Legacy Support Enable and Secure Boot Disable;
Advanced | Boot Options | UEFI Boot Order - unchecked;
Advanced | Boot Options | Legacy Boot Order - checked.
I prepared the media with "Acronis Bootable Media Creator" or with Rufus v3.4 from the Acronis.iso image (tried Rufus in all possible modes). 

The larger USB sticks simply do not appear in the Legacy Boot Order list. 

Would appreciate any advice...

[panksnet · Accepted solution]

Solved, almost. The stumble block was in the utilities used for creating the bootable flash drives.
1. It turned out that the Acronis-2018 own "Bootable Media Creator" not always attribute the partition being created as "Active". It creates the main partition with the right content but makes it "Active" for the 2GB flash drives only, and usually neglects doing so for the drives of a larger size (4GB, 8GB). The HP Pro computer, unlike my older machines, does not boot if the bootable partition lacks the "Active" attribute.
After assigning the "Active" attribute with the diskpart utility, the flash sticks made in the Bootable Media Creator became totally operational.
2. The flash drive created with Rufus V3.4 out of the Acronis.iso
image file only worked if I used the "DD-mode" in Rufus. I didn't manage to boot from the flash drives created in the "ISO-mode", despite that the "Active" attribute was present in this case.