Issue with the JSON object serialize

Question

I am retrieving the list of all the fields and trying to serialize. Using the code snippet below allfieldNames contains around 400 fields but Data >>> contains information related to only 100 random fields. Any idea ?

Map <String, Schema.SObjectType> schemaMap = Schema.getGlobalDescribe();
Map <String, Schema.SObjectField> fieldMap = 
schemaMap.get('Case').getDescribe().fields.getMap();
String allfieldNames = ' ';
for(Schema.SObjectField fields : fieldMap.Values()) {

if (allfieldNames == ' ')
{
    allfieldNames = allfieldNames + fields;
}
else
{
    allfieldNames= allfieldNames + ', ' + fields ;
}
}
List<Case> lstCase = Database.query('SELECT '+ allfieldNames + ' FROM Case LIMIT 1');
system.debug('Test>>>>>'+lstCase.size());
if(lstCase.size()==1)
{
system.debug('Data>>>>'+ JSON.serialize( lstCase[0]));
}