Why Apex Schedulers Sometimes Fail: Understanding and Troubleshooting Scheduled Jobs in Salesforce Apex?

global class CallQueueLeadChecker implements Schedulable {
    Datetime dtNow = System.now();

    global void execute(SchedulableContext SC) {
        try {
            checkLead();
        } catch (Exception e) {
            // Log the exception if something goes wrong
            System.debug('Error in CallQueueLeadChecker: ' + e.getMessage());
        }
    }

    public void checkLead() {
        // Query for Leads that match specific criteria
        List<Lead> leadList = [
            SELECT Id, Active_Call_Queue2__c  
            FROM Lead 
            WHERE Active_Call_Queue2__c = FALSE 
            AND No_Call_Queue__c = FALSE 
            AND (NOT Phone LIKE '%wrong%') 
            AND (NOT Phone LIKE '%invalid%') 
            AND (NOT MobilePhone LIKE '%wrong%') 
            AND (NOT MobilePhone LIKE '%invalid%') 
            AND (Phone != null OR MobilePhone != null) 
            AND (Status = 'Open' OR Status = 'Contacted') 
            AND Picked_By__c = null 
            AND Is_Picked__c = FALSE 
            AND (Last_Open_Timestamp_Formula__c < :dtNow OR Last_Open_Timestamp_Formula__c = NULL) 
            ORDER BY Hubspot_Score__c DESC NULLS LAST 
            LIMIT 200
        ];

        // Debug statement to log the size of the lead list
        System.debug('Number of leads fetched: ' + leadList.size());

        // Update each lead in the list
        for (Lead l : leadList) {
            l.Active_Call_Queue2__c = TRUE;
        }

        // Update the leads in Salesforce
        if (!leadList.isEmpty()) {
            update leadList;
            // Log the number of leads updated
            System.debug('Number of leads updated: ' + leadList.size());
        }
    }
}

// Scheduling the job to run every 15 minutes
System.schedule('Call Queue Checker 0', '0 0 * * * ?', new CallQueueLeadChecker());
System.schedule('Call Queue Checker 15', '0 15 * * * ?', new CallQueueLeadChecker());
System.schedule('Call Queue Checker 30', '0 30 * * * ?', new CallQueueLeadChecker());
System.schedule

global class CallQueueLeadChecker implements Schedulable {
    Datetime dtNow = System.now();

    global void execute(SchedulableContext SC) {
        try {
            checkLead();
        } catch (Exception e) {
            // Log the exception details
            System.debug('Exception occurred: ' + e.getMessage());
            // Consider sending an email alert or logging to a custom object for persistent error tracking
        }
    }

    public void checkLead() {
        List<Lead> leadList = new List<Lead>();
        try {
            leadList = [
                SELECT Id, Active_Call_Queue2__c 
                FROM Lead 
                WHERE Active_Call_Queue2__c = FALSE 
                    AND No_Call_Queue__c = FALSE 
                    AND (NOT Phone LIKE '%wrong%') 
                    AND (NOT Phone LIKE '%invalid%') 
                    AND (NOT MobilePhone LIKE '%wrong%') 
                    AND (NOT MobilePhone LIKE '%invalid%') 
                    AND (Phone != null OR MobilePhone != null) 
                    AND (Status = 'Open' OR Status = 'Contacted') 
                    AND Picked_By__c = null 
                    AND Is_Picked__c = FALSE 
                    AND (Last_Open_Timestamp_Formula__c < :dtNow OR Last_Open_Timestamp_Formula__c = NULL) 
                ORDER BY Hubspot_Score__c DESC NULLS LAST 
                LIMIT 200
            ];
            System.debug('Number of leads fetched: ' + leadList.size());
        } catch (QueryException qe) {
            System.debug('Error fetching leads: ' + qe.getMessage());
            // Handle the query exception
        }

        // Process the leads
        if (!leadList.isEmpty()) {
            for (Lead l : leadList) {
                l.Active_Call_Queue2__c = TRUE;
            }
            try {
                update leadList;
                System.debug('Leads updated successfully.');
            } catch (DmlException dmlEx) {
                System.debug('Error updating leads: ' + dmlEx.getMessage());
                // Handle the DML exception
            }
        }
    }
}