Here is some info that may help you. Make sure you don't have a broken spring or cable.

For Side Mounted Springs Only...

You must release the tension on the garage door springs before attempting adjustment or replacement. This is accomplished by opening the garage door as fully as possible until it hits the stop bolt on the track. Hold the garage door in this position by attaching a C-clamp to the track at a point below the lowest door roller (wheel).

This is true even if you have a garage door opener. Just because the door is open does not mean that the spring tension is fully released. Use the emergency release rope to disconnect the opener from the door and push the door up as far as you can without damaging the opener. Then clamp as described above.

If there are safety cables installed, they must be disconnected and removed from the springs. The old spring is attached at two points. One end is a fixed attachment to the garage door track or wood frame. The other end is attached to a pulley through which the garage door cable is threaded. Simply disconnect these two attachments and reattach the new spring in the same way. Be sure not to twist the garage door cable when reattaching the pulley.

While the pulley is disconnected from the spring, give it a quick hard look. If it appears that the pulley bearings are very sloppy and have lots of wobble, replace the pulleys. There is no repair other than replacement. The door will function more smoothly and save you additional work later. And you might as well check the stationary pulleys, too, while you are at it! Makes sense, right?

Once all attachments are completed, test the door once for proper function before reattaching the safety cables. You may find that the new springs may be too "lively" and the door may not close all the way. If that is the case, you will have to look at the way the bare end of the garage door cable is attached to the garage door track, and loosen an inch or two. This end may be simply looped through the holes in the track, or it could be attached by means of a special clip and an S-hook. In either case adjust both sides equally either by shortening the cable or by repositioning the S-hook in a different hole in the track. The springs should visually be in about the same position when extended fully (door in down position).

Of course, if you do not have enough tension in the springs, and want to make manual opening of the door easier, you may adjust the garage door cable into a tighter position also.