can protein modifications affect immunodetection?

[Scientist]

Hi all!
im trying to detect a  protein in plants Crude extract using
polyclonal Abs. These Abs were obtained from rat sorum, The protein
used to innolculate the rats was expressed in E.coli (thus lacking the
typical postrasductional modifications occurring in plants) .
Untill only no positve results were obtained.
When recombinant form is used as positive control everthing works
well.
So the possibilities are :
a) native form is absent in the plant.
b) the protien is present but in low concetration
c) native form is modified in someway , thus the abs cannot recognize
the epitopes.

have anyone a suggestions for these items?
thanks a lot