Thinking about the matter. I am not really sure the mission chain is broken or not being followed correctly.
The chain is involved, it has a series of NPCs each one requiring different things and when each one is satisfied they pass you on to the next one in line.
You have 2 follow the chain - if you don't the NPCs will either ignore you or poacher James will shoot you (and I think the scientist won't respond if there are other missions involving her that haven't been started nor completed).
You get all the stuff for Poacher James - that mermoth parts & finally the Rextelum chitin fragments, he then gives you an introduction to his brother Poacher John - visit him and he will ask for Rex Aeth & Sabu stuff; give you 4 mission tokens when you collect them.
I am doing the Rextelum Mission and you end up shooting Aeths & Sabus whether you want to or not - but its a good way 2 get two benefits from the one mission.
When you have them go & see Poacher John & he will give you the 4 mission tokens. Then asks you to come back next day if you get any more.
This is when it got vague for me - there are always high level mobs nearby and they will attack and also those extremely irritating Tantillion creatures, not much damage but jumping up & down & aggravatingly bark bark barking at me. I was trying to follow the script, while staying alive. I think I did, but turned up next day & no response.
The wiki says mission is repeatable - but so far for me it isnt. Is there any missing step or response?