2" screws are way to short.
What type screws did you use and did you predrill and counter sink the holes?
If there to short there not going to hold, I would have used ceramic coated bugle head 3" deck screws
If you did not predill and counter sink the screw will try and lift up on the tread and have no pulling power.
They sell a special dril bit with a counter sink made right into it so it can be done in one pass. If you sink it to the correct depth you can go back and add wooden plugs and sand them smooth.