Originally Posted by
GrumpyDiver
Something worth checking would be to see if this is a "feature" or setting of your camera. Check your manual to see if there is a maximum time that the shutter can remain open when using the bulb setting. On my D90, the camera is hard-wired to close the shutter after a 30 minute exposure, regardless of what your cable release.
Weak batteries, either in the camera or in the transmitter or receiver could also be the cause. Try using fully charged or fresh batteries to see if that makes any difference.
If you know someone else with a camera that the unit works with, you could try it out with it, and if you get the same result, chances are that the unit is defective. If not, your camera is more than likely the problem.