Dear Sina Rostami thank you for your helping 

You might play around with temperature during the binding of the aptamer as well as the time of incubation. Too warm and too long (4 hr, 37C) of a time might cause the aptamer to become internalized. Too cold and too short (15 min, 5C) will lead to little binding. I assume that you are binding to surface exposed proteins. 

Another issue - do you anneal your aptamer after you retrieve it from the refrigerator before conducting your cell binding experiments?

 Dear David

thank for your helping

I do cell SELEX and Isolated some Aptamer, after cloning I select one sequence  from each family amplified them by PCR then I do ELONA and flow cytometry finally I select some of them and synthesize them by DNA synthesizer (I  select sequence that have same result in flow cytometry and ELONA) but now I do not see any difference between library fluorescent intensity and selected sequence. incubation period and temperature is like ex experiment. 

Is it possible that some non-specific binding affect the results of control? What buffer did you used for binding and flow? By the way, is the affinity of aptamer binding to cells low?