OSPD10 introspection failed on ipxe boot : Kernal panic - not syncing: VFS: Unabled to mount root fs on unknown-block(0,0)

steps :
1. installed undercloud OSP-D 10
2. import 13 servers
3. introspection failed 6 servers [ tried mulitple times but only 6 failed]

screen hang on the error massge:
Kernal panic - not syncing: VFS: Unabled to mount root fs on unknown-block(0,0).

undercloud :
16GM RAM/ 8 vCPU / 100GB HDD on root

any comment will be appreicated