Do you know that BL21 DE3 is not recA deficient and that any plasmid
that you put in can undergo recombination. I had that problem (Actually
the opposite-no expression) and when I checked the plasmids isolated
from BL21 DE3 the insert was gone. May be your plasmid had DNA
rearrangment. Check plasmid, if this is true then switch over to
another host like JM109 DE3, NovaBlue DE3 etc.
Hope this helps.