Make sure that your bone names match the name used for the .GR2 file and the resource.
Eg. for a mesh with name WPN_DB1, the bone hierarchy should be:

Dummy_WPN_DB1
- Dummy_Arc_A
- Dummy_Arc_B
- Dummy_ArrowFX
- Dummy_FX
- Dummy_String
- WPN_DB1
- - Dummy_Attachment


Also make sure that the "WPN_DB1", "Dummy_FX", "Dummy_ArrowFX" and "Dummy_Attachment" bones have no vertex influences at all, as they're only used for binding by the engine.

Bone "Dummy_ArrowFX" is used for projectile bindings, "Dummy_Attachment" is the one the engine uses to bind the mesh to the character, "Dummy_FX" is a bone for particle effects, "WPN_DB1" is the "real root" bone.

Dummy_Arc_A, Dummy_Arc_B and Dummy_String are used for animations (see eg. the WPN_Human_Normal_Bow_A_Anim1.GR2)