I try to understand if two proteins interact by using Biacore T200. The ligand is 35kDa and the analyte 24kDa. At an immobilization level of 2000RU of the ligand I get a signal of 400RU of the analyte when using 500nM of it. At an immobilization level of 700RU (the only parameter that changes) I get no signal at all. What does this mean?