There''s not quite enough information here. It sounds like there is either play between the rudder post and its bearings (bushings) or play between the blade and the rudder post. Some play between the rudder post and the bearings or rudder tube is fairly normal. You do need to try to determine the cause because if it is being caused by the post cutting through the glass of the rudder tube then it will weaken the rudder tube over time. More likely than not there are plastic bushings of some kind and they have simply worn or shrunk a little.
If the play is between the rudder post and its fins and the core of the rudder, that is much more serious and should be addressed.