Don't get hung up on the whole undercover agent story line. The game looks at your current allegiance not what you originally chose. Lunar Ronin has it correct. If your allegiance is Resistance (regardless of how you chose in the tutorial) you cannot access Loyalist Power arcs. If you are Loyalist, you cannot access Resistance Crusader arcs. If you want to complete all of the story arcs in a zone you need to switch allegiance at least once.
To the OP, you were likely a Loyalist when you were sent to see Dr. Hetzfeld and subsequently switched to Resistance which locked you out from his story. If you switch back to Loyalist you will be able to get missions from Hetzfeld, Bobcat, and Neuron.
These are only some of the pitfalls of running story arcs in Praetoria (1-20). If you are not careful you can easily out level contacts in each zone (I turn off XP at level 10 in Nova, 15 in Imperial, and 20 in Neutropolis to avoid out leveling contacts). You can also remove contacts completely if you do story arcs in the wrong order and thus miss out on their missions. Run the Resistance Warden arcs (up to Seer 1381) in Imperial City before completing Interrogator Kang's last mission and don't take any missions from Calvin Scott until after you have completed Crow and Helix story arcs in Neutropolis (Resistance Crusader).