If there is a small cleavage required for the protein maturation, very likely your tag is cleaved and degraded. Change the N-terminal fusion to C- term or vice versa. Or use the specific antibody instead of GFP or other tag Abs.
anti-GFP from Roche is very specific For you protein, you may want to consider adding a Kozak sequence in front of your fusion, if you are using the pEGFP-N vector. The truth is in front of the free GFP in most cloning vector there is a Kozak sequence and that's why the free GFP always has great expression, but if you are using the pEGFP-N vector, because the MCS is in front of the Kozak sequence, therefore your insert will have no Kozak sequence.