you need to create your symbol (assign an instance name if that's ever needed) and only then add additional keyframes. that's the only way to guarantee your symbol will remain a symbol (and have the same instance name) in the other keyframes.
if you make any changes to that symbol in any of the keyframes, those changes will NOT be applied to any other keyframe.
So no way to fix it? Select whole timeline and my objects and press f8 to create symbol on all of there keyframes or something like that?
that won't work. i know of no shortcut to fixing that after the fact.
the easiest (though not necessarily easy) way to fix that is to move that symbol onto its own layer (if it's not already the only object on its layer). clear all keyframes except the first one that contains that symbol. make whatever changes to your symbol you want to show in the other keyframes and only then add those other keyframes.
in each keyframe you can make more changes, but those changes will not apply to any other keyframe UNLESS you make symbol changes and after those changes then add keyframes further down (not up) the timeline.