for every log file there is a .workData file in the appropriate log files folder which contains information about which logs have already been sent. You could try removing this file and see if all data is pushed to the reporter then. I have actually never tried, so I can't guarantee what happens.
I think the most official way would be to download all logs and manually push them into Web Reporter via FTP.
Building on Andre's suggestion, here is a method to manually push all the logs to Web Reporter:
# Access logs are named in the following fashion:
# The below command will allow you to upload directly from the Web Gateway to Web Reporter, you will need to fill in the blanks for the [YYMMDDhhmm]:
lftp -u [LOGSOURCE-USERNAME]:[LOGSOURCE-PASSWORD] -e 'set ftp:use-feat off; mput /opt/mwg/log/user-defined-logs/access.log/access[YYMMDDhhmm]*.log*; quit' -p 9121 [WEB-REPORTER_IP]
# Below is an example upload command, this would upload access logs from July 2010 to 10.1.1.1 on port 9121:
lftp -u webwasher:webwasher -e 'set ftp:use-feat off; mput /opt/mwg/log/user-defined-logs/access.log/access1007*.log*; quit' -p 9121 10.1.1.1