Losing an advantage, failures and mistakes, increased pressure and ticking clocks, loss of control

A neighbor removing their name from a complaint they helped the character lodge with the city
A co-worker who helped influence a difficult boss quitting
Losing an influential voice for funding at the university where the character works
The character's co-council removing themselves from a case
Losing a connection at an influential newspaper or law enforcement office
A detective who supplied crucial information being kicked off a case
A business partner or co-founder deciding to leave the company
A friend who was a constant source of support and encouragement passing away
A spouse reversing their stance on a parenting issue and pitting themselves against the character
A sibling losing their resolve and giving in to family pressure
A political peer crossing the aisle to join another party
A teammate being traded to another sports team
Losing a talented assistant in an office shuffle
A co-worker being offered a too-good-to-turn-down job at another company
A fair-weather friend changing social groups when the character becomes less popular at school

Having more work than one person can handle
Taking on new responsibilities
Having to find alternative sources of information
No longer being in the loop
Losing confidence in the mission or goal
Having no one to talk to about a specific problem or challenge
Losing the person's connections and skills 
Missing the friendship
Having to keep up the spirits of those who remain
Having to act confident to put on a show of strength
Having no one to safely share misgivings and doubts with 
Having to hide disappointment and act happy for the ally in their new opportunity
Needing to build new alliances