Did you first run the command “isohybrid” on the iso?
The NET iso comes ready to install on a CD. The standards for booting a CD are different from those for booting a disk.
Some BIOS will work with the CD method used on a USB. Others won’t. The “isohybrid” command fixes that, so it should work with all sufficiently modern BIOS. You might have to install the “syslinux” package to get the “isohybrid” command.
If you are dealing with an older computer, the BIOS might not even support booting from USB. I’ve been able to get around this on a couple of older systems with the PLOP boot manager (google for it).