I cant find why this script sometimes works and sometimes doesn't

Question

I took over someone's else backup script and change it into something a bit more robust. Initially, it would create a single huge tar.gz file (280GB+), but I change it so it would create many smaller tar.gz files (one tar.gz per folder). If I run the script manually, it does the compression but the log sometimes fails. I need to filter the folder by date and send that by email. Can't figure out why it fails. So I removed the date and used the wildcard so I get the entire folder with previous tar.gz files. Is not what I want but it works. For each email I get I check by eye if all folders are there.

I schedule it on a crontab job to do it every day (* 19 * * * /home/user/backup.sh). Sometimes only a few folders are compressed and the others are not. Sometimes doesn't do the compression at all and the logs come empty and then I check if the files are there, if they are accessible, and they are. I don't understand why doesn't work. It's driving me mad.

Any help?

The script:

#/bin/bash

find /home/bkp/ -mtime +4 -type f -exec rm {} \;

# Gets the date to calculate the time spent.
BEFORE="$(date +%s)"

mount -a

IS_MOUNTED=`mount | grep /mnt/poa`

if [  -z "$IS_MOUNTED" ]; then

  echo "ERROR" | mutt -s "Backup Server" [email protected]
  echo "ERROR" | mutt -s "Backup Server" [email protected]

  exit 1

else
  # This loop takes any non-folder files and moves them into a specific folder.
  for PATH_TO_FILE in "/mnt/poa"/*
    do
      # Is a file?
      if [[ -f $PATH_TO_FILE ]]; then
          # Ignore Thumbs.db
          if [[ $PATH_TO_FILE != "/mnt/poa/Thumbs.db" ]]; then
            mv "$PATH_TO_FILE" "/mnt/poa/Random/"
          fi
      fi
  done

  # Gets the date.
  DATE=`date +%d_%h_%y`

  #echo $DATE

  # This loop does the backup.
  for PATH_TO_FILE in "/mnt/poa"/*
    do
      # Is a folder? 
      if [[ -d $PATH_TO_FILE ]]; then
        FOLDER_NAME=$(basename "$PATH_TO_FILE")
        #echo "Path      --> $PATH_TO_FILE"
        #echo "Basename  --> $FOLDER_NAME"
        #echo "Full name --> ${FOLDER_NAME}__${DATE}.tar.gz"
        tar -czcvf /home/bkp/${FOLDER_NAME}__${DATE}.tar.gz /mnt/poa/${FOLDER_NAME}
        #echo "/home/bkp/${FOLDER_NAME}__${DATE}.tar.gz /mnt/poa/${FOLDER_NAME}"
      fi
  done

  # Log file
  LOG=/var/log/backup/${DATE}.log
  echo "# Backup Log #                  ">> $LOG
  echo "" >> $LOG
  echo "" >> $LOG
  echo " -- Files :" >> $LOG
  echo "" >> $LOG
  # Fails most of the time
  #du -sch /home/bkp/*_${DATE}.tar.gz >> $LOG
  du -sch /home/bkp/*.tar.gz >> $LOG
  echo "" >> $LOG
  echo "" >> $LOG

  # Math for time spent on this backup.
  AFTER="$(date +%s)"
  TIME_SPENT="$(expr $AFTER - $BEFORE)"
  HOURS=$(($TIME_SPENT / 3600))
  TIME_SPENT=$(($TIME_SPENT - $HOURS * 3600))
  MINUTES=$(($TIME_SPENT / 60))
  SECONDS=$(($TIME_SPENT - $MINUTES * 60))
  echo "Time spent" : "$HOURS HOURS $MINUTES MINUTES $SECONDS SECONDS ">> $LOG
  cat /var/log/backup/${DATE}.log | mutt -s "Backup Server" [email protected]
  cat /var/log/backup/${DATE}.log | mutt -s "Backup Server" [email protected]
  cat /var/log/backup/${DATE}.log | mutt -s "Backup Server" [email protected]
fi