I built a three shelf bookshelf for my daughter and used Elmer's wood filler in order to fill in the screw holes. The container for the wood filler noted it was stainable. I put the first coat of stain on last night and the filler did not take the stain at all. I wanted to see if anyone had any suggestions on how to 1) fix this and 2) a different product to try? Any help is appreciated, thanks!