Diagnosis
This is typically a failed HDMI port on the main board or a corrupted EDID handshake specific to that port. The PX701-4K has two HDMI ports (HDMI 1 & 2/HDCP 2.2).
How to Fix It
Force a cold reset of the HDMI subsystem. Power off the projector using the remote. Unplug the power cord from the wall. Press the 'Power' button on the projector chassis 5 times. Wait 2 minutes. Plug the power back in and turn on. Try the device in HDMI 1 again.
If That Didn't Work
The HDMI 1 port may have physically failed. Inspect the port for bent or damaged pins. Try a different, known-good HDMI cable. If HDMI 2 consistently works with the same device and cable, the fault is isolated to the HDMI 1 port hardware.
Heads Up
Some streaming devices (like Fire Stick) output a lower-resolution signal when first connected. Go to the projector's Settings > Source and manually select 'HDMI 1'. Sometimes the auto-detect fails. Also, ensure 'HDMI Mode' in settings is not set to a legacy version (like 1.4) incompatible with your device.