If the FBI includes your state criminal history in their files they will often exercise their discretion and follow state expungement orders, but they do not have to do so. As a federal agency the state court order is not binding on them. If you do write the FBI a letter saying, "Please expunge my record," you should expect that request to go into your file. It's also possible that your employer obtained your record through a source other than the FBI - most employers use private background checks.
It is unlikely that you can reopen the case, particularly post-expungement. Consult a local criminal defense lawyer.

