Context:

After you turn in the technology to Elder Hardin (either Vault 22 data or Vault 34 pulse gun). Veronica gets assaulted outside the BoS base by a group of paladins.

Trigger:

After defeating the group of paladins, Veronica starts a conversation.

Effect:

After the dialogue, her following is sporadic, it's completely bugged. She no longer moves, except when you do quick travel and then it's even sporadic as well. She sometimes comes, she sometimes doesn't.

The same thing happened to me. Notice how she might not be carrying any weapons too?

Theres a workaround for this here , I can't remember what it was called but it worked for me!