Hi, and welcome to TFP!
What are you using to test your water? To assist you, we really need a full set of numbers from a good test kit (not strips)
FC
CC
CYA
pH
TA
CH
Salt (you can use strips for this!)
12-40# bags of salt should have raised your salt level by 4313ppm! What are you using to test your salt level??
Shocking is not a one-time event, it is a process. I suspect you are unable to keep chlorine in your pool because something organic is busy eating it up! (Also, with low CYA or stabilizer, your chlorine will burn off in the sun on a daily basis.)
Shock your pool at night, and keep your FC level up until
1) you have no FC loss overnight, and
2) your cc's are <0.5ppm