The trouble with small leaks is that they may be big enough to let the small molecules of gas out, but not big enough to let the dye out. In addition the gas may be so dilute in the air that the sniffer may not find it. The only solution may be to strip the system down and pressure test the individual components but that is a last gasp measure. Some Indies now use a sealing product that will seal anything, but again, as your leak is so small it may not work.