Desorption instead of adsorption from solution using activated charcoal
In such experiments you typically run a blank. So add the homemade activated charcoal to just some water, filter, then titrate that filtrate as a blank. 

There are two possibilities for the increase. 


  
    (1) Your batch of homemade activated charcoal was contaminated with who knows what soluble compound. (This seems the most likely...)
    
    (2) Quite possible that some of the charcoal particulates are small enough to not be trapped by the filter. 
  


Another way to run the experiment would be to add the homemade activated charcoal to just some water, filter, then wash the wet charcoal into the oxalic acid solutions (this would remove soluble compounds from the charcoal). Then dilute all the solutions to some co0nstant volume, filter, and titrate the filtrates. (You should still run a blank.)