You also need to increase your duty cycle. A 15 second exposure every 120 seconds means that you are missing 87% of what's happening. You're going to have to get very lucky to capture something. Maybe see if you can get 15 second exposures every 17 or 18 seconds? Try this in a closet with your wristwatch to see what the minimum interval is. If you set your interval too short, it's going to drop frames (with the interval timer in the loop, the camera won't be able to get the mirror re-opened for the next shot, so it sits it out, waiting for the next interval), and gives you a 50% duty cycle, at best. For instance, for star trail stacks, I set my D800 for a 30 second exposure at 33 second intervals. A 31 second interval guarantees dropped frames, and a 32 second interval sometimes causes frames to be dropped.