First thought would be that there are no records created on the days you aren't getting the reports produced on. On the Actions tab of Solution Builder, there is an option to suppress the report if there are no records. If that is ticked, the email doesn't go.
If you untick it, a blank report will be sent.
I'm sure there are other things that may be wrong but as a simple first check, good place to start.
Thanks,
Andrew