Typically, adjustments do not need to be made until you get above 3,000 feet. My first guess, since you are having trouble across the board, would be the water. Try soaking and cooking the beans in bottled water. Yes, using bicarbonate of soda (aka baking soda) may help to soften the skins, but if the water has a high mineral content, the soda is not likely to help overcome that problem.
Phatch suggested using the pressure cooker, and that is a good approach. Visit my
cooking group , for more information on using the pressure cooker for beans. There is even instructions for the quick soak method, which you may use with or without the PC. You will want to soak the beans, drain off that water and start the cooking process with fresh water. I am at 6,300 ft and have no trouble with cooking dried beans to perfection (not bragging...just stating the fact).