A few more questions to ask:
What is the rating on the power pack? (ie: 5 VA, 20 VA...)
Are you connecting the power pack to the track in one place or many places along the track?
Will the power pack run a train on a short piece of track, like 3 or 4 foot of straght track?
Do you have access to a voltmeter to make a few measurments along the track?
You could have a bad connection, a bad power pack or a loco that doesn't run well. Answering these and the other questions that were asked will help you to isolate the problems.
Don