Even if wood, you may find that there is some clear finish applied on top that would think would have to be completely removed in order for the wood to accept stain.
I recently experimented with a couple of pine curio cabinets I picked up that I thought did not have any topcoat.
I tried applying darker stains to the underside of a removable shelf and had no luck - It did darken, but the stain just dried on the surface leaving a sticky residue 0 it could not penetrate so it appears there was some sort of coating on the previously stained pine.
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.