Verify that the detector is operating properly by testing the thermal background . Note that K or K-prime skies can, at times, get high enough that, when using the wide field of view, saturation occurs in less than the minimum exposure time, precluding the use of that plate scale with those bandpasses. Recalibrate the filter wheels by sending them to their home positions and then resetting them. Check for clouds. Check that the dewar window is free from condensation.
Condensation typically begins forming at the center of the window.
It may appear as a uniform or blotchy circle of reduced sensitivity. Remember that the large sky contribution in the near infrared can make targets seem faint, or even invisible, on single frames before sky subtraction. A quick sky subtraction will allow a more accurate assessment of the camera's performance. Poor seeing also enlarges and distorts images, while at the same time making it more difficult to focus and guide accurately. Recheck the focus.