Amplification of monoclonal by passage through mice

[JLT Tyner]

Assuming the low production values are not the result of genetics and you
have proven by 2 reliable methods that this cell line is a low producer, I
conclude you are not working with a clonal cell line.  That is, you have
an antibody producing cell population within a cell population not
producing antibody.  Try subcloning by limiting dilution to isolate a
homogeneous and higher producing clone.  Re-establish your cell banks.  

J. Tyner
Chicago, Il