Another avenue that you can explore with Bionet is the possibility of protein aggregation - clumping together of the target protein which may be triggered by changes in external conditions e.g. temperature, pH, ionic environment etc. If aggregation had happened sometime after the product was passed through the 60SP02A filter, the larger protein aggregates may be retained by the tighter 90ZA05A during filtration. Alternatively, if the aggregate had managed to pass through the 90ZA05A, you may ask Bionet to scan for its presence at higher molecular weights.