Pool won't hold Chlorine

20,000 Gunite pool
In line chlorinator
Sand filter

Live in North Carolina
Water almost clear
After shocking with 4 pounds of shock at night no residual chlorine in the morning (tried this twice)
All other chemistry is in line...except Chlorine which is non existent...something is consuming it
At 4 pounds a shot, its starting to get expensive to have no success

This morning I've read a bit about using liquid bleach to get me to a point where my chlorinator can maintain the chlorine level...just can't seem to get over the hump! Thoughts?