Lost messages in a bag

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I have a rosbag file where I recorded Image messages

For some reason that I don't understand when I do in one terminal

rqt_bag the_bag.bag

and I activate publish in the interface and then in other terminal I do

  rostopic echo /the_image/header

I have the following two problems:

1. I never get what is supposed to be the first message that I published
2. I never get the same message. Sometimes it echos from the fourth message, sometimes from the fifth, sometimes the sixth

So, why is this happening and how can I inspect the rosbag to make sure that all messages are there?