In todays automotive world their is a product which I believed was developed during the early days of the space program and is called RTV (room temperature vulcanized) silicone. This is a silocone base elastomer which is commonly used as a sealant or an adhesive. Some people love to use it everywhere even more than JB weld. My guess is someone put a niece bead of RTV along the edge of your floorboard that is in contact the floor frame. As a starter, I would run a knife between the wood and the frame. This should help to loosen the floorboard if this is what is holding it.