Hi there! I'm afraid I'm not super experienced in GameMaker, but all the tilesets should be 16x16 pixels. If you're using the first version of GameMaker I believe tilesets are imported as Backgrounds, and there's a tick box on the left above Texture Settings called 'Use as tile set' that should cut it up into tiles.
Apologises if this doesn't fix it or isn't the issue, let me know if it's a specific tileset doing this and I can try and fix it! In terms of GameMaker I might not be much help...