1. Is the USB cable in the right port
On most panels the touch signal goes over USB, and the port marked 'touch' is not the same as the service port. A cable in the wrong port gives you a perfect picture and no touch. Check the label, not just that it is plugged in.
2. Is the operating system seeing it
Windows: Device Manager, Human Interface Devices. Android: Settings, Input devices. If the device is not listed, the cable or the port is the issue. If it is listed but the cursor does not move, it is a calibration or driver issue.
3. Has it been calibrated for this resolution
A panel that worked at 1080p and now runs at 4K can lose touch alignment around the edges. Run the calibration routine after any resolution change. This is the single most common cause of 'touch is offset by 30 mm'.
4. Is something covering the frame
Infrared panels have emitters in the bezel. A decorative frame, a sticker, a cable clip, or a dust line along the bottom edge will block one axis. Wipe the inside of the bezel with a dry cloth and look for anything touching it.
5. Is a second input device fighting it
A wireless mouse dongle, a USB hub, or a second touch device can make the cursor jump. Unplug everything except the touch cable and test.
6. Is the glass wet or very cold
Capacitive touch behaves badly with a film of water or condensation. In a cold room brought up to temperature quickly, the glass can fog for twenty minutes. This is not a fault; it resolves itself.
7. Is the panel in the right touch mode
Many panels have a mode switch for Windows, Android, Mac, and sometimes Linux. The wrong mode gives partial or rotated touch. Check it before assuming hardware failure.
When to stop and call
If the device does not appear in Device Manager with a different known-good cable, and the same cable works on another panel, it is the panel's touch board. At that point a remote session with the supplier saves a site visit. Send the serial number, the OS version, and a photo of the port you used.