Probably the bracket clamping screw has worked loose.
Make sure the bracket is centred so it isn't pushing the barrel to one side or the other and nip the screw up tight again.
There should often be a tiny amount of play where the barrel passes through the bracket, but the bracket itself should not be free to move.
Your eyes are remarkably sensitive, if you take the scope off and look along the length of the barrel you can generally tell if it is correctly aligned.
If you loosen the clamp off the barrel will often want to sit in a natural alignment with the breech block.
This is the method I have used successfully over many years