You need to determine what type of glue was used. Go to the local tile supply place and ask them for their recommended stripper / softener for your specific glue. MEK works sometimes (Methyl Ethel Keytone) alcohol works on some glue, paint thinner mineral spirits, turpentine, vinegar, sometimes even comet and ammonia (BUT WATCH OUT) do this in a well ventilated area bleach and ammonia products are harmful to adults and children in confined spaces.