The single most common "why doesn't this input work" call in industrial automation isn't a broken wire — it's a sourcing module wired to a sourcing sensor, and neither side able to complete the loop.
This is the exact same underlying idea as NPN vs. PNP sensors — but looked at from the PLC module's side of the wire instead of the field device's side. A digital input (or output) module doesn't just accept any 24VDC signal — it's internally built to provide one specific half of the circuit, and it needs the field device on the other end to provide the other half. Get that pairing backward, and the input never sees a valid signal at all — regardless of whether the sensor itself is working perfectly.