1) does it have an RVA built into the ASA?
2)have you had the ASA off?
It could be the spring is shimmed/rva is too far in and stopping the hammer from catching on the sear.
3)have you checked the sear or the catch on the hammer for damage?
As for the screws I would try the local hardware store if you want them. I ran my SB for years without them, just rember to keep the frame screw tight! Hope this helps.