1. Battery is full.
2. There is lots of space left in the SD card.
3. Camera is put to MANUAL mode.
4. Camera is put in BULB mode.
5. Disable mirror lock, if any.
6. Make sure power-saving is off since camera can automatically turn off after while making it impossible to connect to.
Try this command to capture, it will generate a log that could be useful for upstream libgphoto2 developers.