Top point 50 nM and the curve is leveling out: you reached the Rmax of the system.
ka is approaching limits: I don't know this warning but it is possible that the calculated ka is that high that is is outside the normal range which can be measured with the machine. Although the curves doesn't look that bad.
For a nice sensorgram, good fit and accurate numbers is is not necessary to inject to Rmax. Try a different range (0.05 - 5 nM).