may be can show us some photo? So we have better understanding about how flat it is?
If the flash is too strong, or direct flash. Sometime can cause these "flatness" problem. That's why some use bounce card, ceiling bounce or diffuser.
You migth like to read what I wrote to another fellow forummer regarding flash and why it can sometime produce rather flat flash shots. Just click this link and refer to my message #5. It might shed some light and I am sure others will have more to add as well to it.