sed stdin behaviour

using sed for a direct sdtin treament of data, i expect that a line is treated after the enter. This is waht occur on cygwin and linux that i test but on my AIX, a second new line is needed to treat the line. I try with option -u for avoiding a buffer latency without modification in behaviour. Is there an option or parameter to let the posix AIX sed to work like on other OS

sample:

> sed 's/foo/bar/'
like my foo           (enter)
but not the lazy dog  (enter)
like my bar           (stdout)