So you definitely made the song in the past and played it to the scarecrow, and then played it again to it in the future (just making sure)? I assume you're trying to get the golden skulltulas in the Fire Temple. I remember trying to summon him is frustrating because sometimes the game just doesn't like where you are standing when you play the song. Sometimes to get the scarecrow to summon in the Fire Temple I've had to play the song 5-10 times, moving around on the platform where you're standing and alternating facing towards where he summons and facing away. If you're sure you set up the song correctly, then just keep trying. The game is much less than perfect when it comes to summoning the scarecrow even when you've done everything correctly.