How can i do more weight with barbells? im really weak when it comes to barbells?

Your using different muscle sets when you go from a dumbell to a barbell so can't expect that you can double the weight when you move to a barbell even though you are using 2 arms. Your other muscles that are used to lifting dumbell weight need to be conditioned to the extra weight on the barbell. Keep doing your routine with the barbells adding weight but fewer reps each time. You want to lift to exhaustion in order to break down and rebuild your muscle mass. Also don't worry too much about competing with the amount of weight the other guys are lifting - focus on your own program.