How can I modify standard SAP reports when my modification assistant isn't working?

I've never had a problem with the modification assistant not working. To modify a standard SAP report, you need a modification key from OSS. You should always use the modification assistant to record your amendments. Think carefully about whether there is another way of acheiving what you want -- perhaps using SUBMIT...EXPORTING LIST TO MEMORY, a couple of standard reports, and then merging them into yours.

You could copy the standard report, and modify the copy, but then you put yourself outside of SAP's maintenance and bug-fix track -- you might end up having to re-copy to incorporate new features. I'd always recommend modification over copying.