If yours is a genuine Apple 30 pin connector it may have two tabs on either side that must be pressed to release its internal locking tabs, so press hard, and pull hard.
Make every effort to pull it straight out, not at an angle, and have a soft "backstop" such as a pillow in case the iPod goes flying out of your other hand when the connector finally lets loose.
If you can't get a good grip on the connector with your fingers you may need to use a pair of straight slip-joint pliers. Using a tool like that may destroy the connector in an effort to remove it, but you may not have any other choice.