Are you on firm ground? Many streets and sidewalks in New York City do not provide a solid surface for long exposures. What? Why? If there is a subway line under the sidewalk then the ground will move every time a subway train passes. Also if you have your tripod set up close to a road, the passing of a large vehicle is likely to shake the ground. If you are on the Brooklyn Bridge you will never get a tack sharp image with a long exposure because it's a suspension bridge with cars moving below. Balconies are also problematic as they often have some give; if someone walks next to you while you take a long exposure it will vibrate the camera. If you are on dirt, make sure to push your tripod all the way into the ground until it stops.
Windy, even slightly windy conditions, can also cause problems. In this situation you will need a very sturdy tripod or sand bags to hold down your camera.
Another cause of blurry photos while using a tripod is you. The simple act of pushing the button may cause the camera to move. To resolve this problem you can use the self timer function on your camera, a cable release or a wireless release button.
The last cause may be the object you are photographing. Look closely to see whether or not it's moving. Buildings sway, clouds move, as do bridges and seemingly solid objects. The longer your shutter speed, the more likely you will see blur.