Trying to find SMS messages from a certain time frame

I was wondering if anyone knows how to find deleted text messages (SMS messages) from a certain time frame? I am able to retreive deleted messages from March and some of April, but I cant find the ones from the last week of April until the third week of May. Any ideas? I found a really cool free app that extracts the SMS mesages from the backup file, but its weird, there is no file for the dates above, just March, 3 weeks in April and then just the last few days of this week in May. Like four whole weeks is missing. Any ideas would be great. Thanks

~richelle