"Are you sure all the wires are connected, USB and power?" (Relating to a scanner.)
"Yes, I've checked several times."
get there, USB is firmly connected but the power connector was hanging like 2cm belown the desk, clearly visible when you looked at the back of the scanner.
At that same trip dropped in to check a complaint about a broken DVD-drive. Turns out it didn't read DVDs because it was a CD-drive.