Is the signal you get in correlation to the concentration saturable? Do you use a buffer containing Sodium chloride and polysobate as in biacore's HBS-EP buffer to limit unspecific binding? The ligand is quite small so I don't know how relevant transport limitation is. Another effect can be the influence of shearing forces on the protein (high shearing = conformational change that inactivates the protein). What happens if you measure at 10 and 20 ul/min. Is there some kind of flow-barrier where you loose all binding activity?
If you get nice results at flowrates around 10 - 20 ul/min I would go for this rates.