Improper instrument installations can lead to:
Noise from swinging valves and impulse lines
Zero-offsets which can inhibit proper control, but
    most detrimentally can defeat trips (in safety systems).
Valves with structurally stressed mounting can fail, stick, and have high wear
Improper analyzer installations create long deadtime, creating poor control
    situations.
Installation inspection requires on-site review.

Control Block issues will degrade control:
The wrong blocks may have been chosen
The control modules or blocks within the control module may be
      processing in the wrong order.
Control initialization can be compromised by the configuration
Bad readings may not be properly handled
Transfer to/from Automatic may not be properly configured

Control Block problems can be identified remotely

Operator Understanding is key to keeping controls in Automatic
If the operator believes they can do better than the scheme, then
    they do not understand the scheme or the scheme has problems
If the operator cannot place the scheme in Automatic effortlessly,
    then he needs training, or the scheme needs to be fixed
Most operators and process engineers do not understand that levels,
    by design should float between a high and a low, to dampen changes.
Control scheme detail graphics may be required

Valve, damper and positioner issues
When the final control element does not respond as expected
control is degraded. Valve stiction is one example.
Modern smart positioners can be tuned with excessive gain
and create valve overshoot
Rapid process changes can also cause rapid valve movement

Process issues and the process design may require control changes.
Many reaction systems have time variant or temperature variant control gains,
on-line gain, or integral tuning changes may be required.
Process disturbances often overwhelm the controlled variable,
feed-forward control or control output calculations may be needed,