Losing an advantage, loss of control, duty and responsibility, increased pressure and ticking clocks

A witness to a crime becoming incapacitated or dying
Learning information about a promising witness that makes them unsuitable (a criminal record, a penchant for lying, a bias that will taint their testimony, etc.)
A witness refusing to come forward because it will get them into trouble with a spouse or parent
A sibling declining to get involved because they have a reputation to protect
Failing to get contact information from a pedestrian who witnessed an accident
A witness being intimidated into backing out of a case
Mitigating circumstances that will make a witness seem unreliable to a jury
A judge ruling that a child witness cannot provide testimony 
A witness being deemed mentally unfit for court
A witness abruptly changing their testimony
A witness disappearing without warning
A co-worker refusing to testify because they've been told by the boss to stay out of it

Pleading with someone to step up and say something
Making promises to secure someone's help
Having to hunt down evidence of wrongdoing
Talking to anyone who might have seen something
Missing work or school to try and collect evidence 
Seeking out people who may have recorded what happened on their phones
Asking nearby businesses if they have surveillance video 
Pleading with police to investigate certain people or locations
Needing to find another witness
Having to build a stronger defense
The character needing to testify on their own behalf
Filing a legal appeal
Asking for a continuance