Problems with Rmax and Fitting curves for IgG4 antibody

Hello,

I'm a new user of the Biacore T200 and since I have a really low experience I'm looking for some suggestions/explanations of some strange events I'm encountering to test the KD of two Ig4 bivalent antibodies.
I already knew that the are really strong binders and their neutralizing activity against their cognate antigen is good (tested with cellualr bioassay).

I'm actually doing a multiple cycles kinetics with the following parameters:
CHIP used: Serie S protein A
Captured antibody: 60s at 5ul/min
Sample: contact time 360s at 50ul/min
Regeneration: Glycine pH1.5 30s at 30ul/min
Dissociation time: 600s (for the Ab 1G10) 720s (for the Ab 13A1)

I diluted my antibodies in order to have around 20RUs as maximal response with the analytes concentrations.
The problem is that with such a low concentration of antibody coated on the chip I cannot explain how can I reach this kind of Rmax. The theoretical Rmax should be around 3.
Atibody MW 144kDa, Analyte MW 25.8kDa

Here I attach the dissociation curves with the kinetic informations obtained with the bivalent fitting.
Antibody 1G10

Antibody 13A1


Please let me know if you need more informations.
Thank you very much