Inconsistent R_Max Between "Experimental Replicates"

Good evening,

I have an SA chip with a biotinylated protein immobilized. The expected R_max is around 950 RU. The first run of this sample at the highest concentration was ~130 RU (100% protein). The ligand concentration was immobilized at 130 µg/mL, reaching its expected R_ligand of 1700 RU after the first pulse...

Where things get interesting is that every subsequent replicate assay, has had a 100% binding set of samples that is slowly decreasing.

Over the past week the 100% binding control has dropped from 130 -> 100 --> 80 --> 40 RU. We have a regeneration solution of buffer at pH 5 instead of 7.4, just to moderately disrupt electrostatics between the ligand and analyte.
We know that no protein remains, as the response in the "within-experiment" replicates are consistent. I.E. All samples will have a minimum deviation at the same concentration.

Has anyone experienced this before, if so do you have any tips on how to control it?

We are aware of the possible supersaturation of ligand on the chip (indicating the ~14% ligand availability), but I don't think this accounts for the slow decrease in R_max over time...to what is now 4% of functional ligand.

Thanks.

If I understand correctly you have immobilized 1700 RU ligand which in combination with the analyte could give an Rmax of 950 RU. You observe ~130 RU at the experimental start and over the days this drops to 40 RU.
Some thoughts:
How is the control stored? Could be degrading, freeze/thaw cycles? Sometimes it is better to store at 4ºC when using within 1-3 days or aliquot in small volumes.
Storage of low concentration analyte solutions could lower the actual concentration by sticking to the vial wall.
(Any enzymatic reactions?)
What is the run time of the experiment and are the 100% controls at the start and end? And are your samples randomized. If not, injection of replicates after each other can hide regeneration problems such as slow degeneration of the ligand.

Kind regards
Arnoud