SFS eligibility

What are the exact requirements to be elegible to a SFS drop. I dont mean being on the quest. Thats a given.

Do you need to be there when Marrowgar gets killed? Can you join later? Are there different rules for the LK shard?

It happens that I join a raid somewhere in the middle near the end of a reset. Sometimes I can get SFSs, sometimes I dont. And it wasnt LM trying to scam me (ok, maybe). He even shared his screen on discord to show me that he couldnt assign the drop to me, whilst I got a message in my client " You could never use that item"

I know that I can get shards, I am on the quest, I already have 45 of them. Is it because I stopped playing on Icecrown for quite sometime and they chaged something about the quest? I am just trying to understand the system.