[SalesForce] Apex Batch runs constructor but does not execute start method

I´m trying to run a Batch to delete some 'expired' Cases. In the test method when I execute the Batch the class constructor runs but the batch start method never runs.

The batch class:

global class ISSP_AMC_CaseCleanUpBatch implements Database.Batchable<sObject> {

 private Date dateToDeleteTo;

 global ISSP_AMC_CaseCleanUpBatch(){

    AMC_Configuration__c amcConfiguration = AMC_Configuration__c.getInstance();
    Integer numberOfDays = Integer.valueOf(amcConfiguration.Case_Number_Of_Days_To_Delete_To__c);

    dateToDeleteTo = Date.today() - numberOfDays;
 }

 global Database.QueryLocator start(Database.BatchableContext BC) {
    // get the RecordType Id
    Id recordTypeId = Schema.SObjectType.Case.getRecordTypeInfosByName().get('Airline Coding Application').getRecordTypeId();

    // Query Case Object
    return Database.getQueryLocator([
            SELECT id
            FROM Case
            WHERE Id IN (
                    SELECT Case__c
                    FROM Participation_Online_Application__c
                    WHERE LastModifiedDate < :dateToDeleteTo
            )
            AND Status = 'Open'
            AND RecordTypeId = :recordTypeId
    ]);

 }

 global void execute(Database.BatchableContext BC, List<Case> scope) {

    List <Participation_Online_Application__c> participationList = [SELECT Id FROM Participation_Online_Application__c WHERE Case__c IN :scope];
    if ( scope.size() > 0){
        delete scope;
    }

    if ( participationList.size() > 0 ){
        delete participationList;
    }

 }

 global void finish(Database.BatchableContext BC) {
 }
}

The Test Class part that executes the Batch after the data is created in a Data Factory:

    Test.startTest();
        ISSP_AMC_CaseCleanUpBatch cleanUpBatch = new ISSP_AMC_CaseCleanUpBatch();
        Database.executeBatch(cleanUpBatch);

        List<Case> batchedCaseList = [SELECT id FROM Case];
        List<Participation_Online_Application__c> batchedParticipationOnlineApplications = [SELECT Id FROM Participation_Online_Application__c ];
        List<Participant_Online_App_Requirement__c> batchedParticipantOnlineAppRequirements = [SELECT Id FROM Participant_Online_App_Requirement__c];

        list<Participation_Online_Application__c> poApp = [select LastModifiedDate from Participation_Online_Application__c];
        System.debug('>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> test ' + poApp);

        System.assertEquals( 0 , batchedCaseList.size());
        System.assertEquals( 0 , batchedParticipantOnlineAppRequirements.size());
        System.assertEquals( 0 , batchedParticipationOnlineApplications.size());

    Test.stopTest();

Any help is appreciated.