You are not getting any precipitate with ammonium sulfate because the overall protein concentration in the culture supernatant is low. I suggest you first bring down the effective volume of your culture supernatant. This can be done in a small batch of 100 ml. Remove 100 ml of the amylase containing supernatant and dialyze against 1 L 25mM Tris buffer containing 10 mM MgCl2 and glycerol at 50% v/v at 4C for 10 h. This will bring down the volume of your filtrate from 100 ml to 34 ml. Subject this dialysate to precipitation with ammonium sulfate at 4C and the process must be carried out for a total time of 48h. You will see a precipitate- it will be light and flaky but the active enzyme will be present in the pellet because amylases are quite stable.