Use galactose couple agarose beads or Fetuin affinity chromatography for purification of lectins.

For activity follow this protocol

Electrophoresis. 1994 Jul;15(7):907-10

A method is described for the direct detection of lectins, agglutinating erythrocytes, on nitrocellulose membranes after Western blotting, thus avoiding protein extraction from specific bands in the gel, followed by agglutination assays. The methodology essentially involves exposing the lectin band on a nitrocellulose strip to trypsinized rabbit erythrocytes (2%, in 0.15 M NaCl) for 30 min at 37 degrees C and then carefully transferring the membrane to saline (4 degrees C) for a few gentle washes and then fixing it in a solution (0.2% glutaraldehyde in 0.15 M NaCl) for 30 min. Later, the membrane is gently washed several times in 0.15 M NaCl containing 10 mM beta-alanine. The lectin band is visualized as a red agglutinated patch. The method is specific for lectins that can agglutinate red blood cells and virtually has no cross reactivity with the various nonlectin proteins tested. Binding of erythrocytes to the lectin band on the nitrocellulose strip can be prevented by specific competing sugars. The method can be applied to screen for the presence of lectins in natural materials and to monitor lectin fractions during purification.

Remember all lectins are not active.

could it be a possibility that your lectin has higher cross-linkage affinity for a different type erythrocyte? Can you increase the concentration and try differing types? Are you storing overnight at room temp or cold? could anything in your protocol be binding to the ligand binding site and competitively inhibiting agglutination?