This is a very broad question. Each table is different and every situation has it's own set of variables.

Maybe we can start on what you are playing. I am guessing NL Hold em? What stakes?

It can be a million things, learning takes time and you have to learn to win consistantly. First you ahve to find your leaks and we can go from there but it isn't always easy. Maybe post hands here so we can walk through one with you. If you post a few hands I am sure me and others here can go over them with you and start the process!