sync problem in stereo image

edit

modified stereo_camera_model.cpp in image_geometry, now I can receive /stereo/disparity message, but still there is problem in image sync when I want to view by: rosrun image_view stereo_view stereo:=/stereo image:=image_rect there is just three gray windows (no image) and a [WARN] message as below any suggestion, please?

[ WARN] [1297946062.739511145]: [stereo_view] Low number of synchronized left/right/disparity triplets received.
Left images received:      104 (topic '/stereo/left/image_rect')
Right images received:     102 (topic '/stereo/right/image_rect')
Disparity images received: 18 (topic '/stereo/disparity')
Synchronized triplets: 0
Possible issues:
               * stereo_image_proc is not running.
                 Does `rosnode info /stereo_view_1297946016943512050` show any connections?
               * The cameras are not synchronized.
                 Try restarting stereo_view with parameter _approximate_sync:=True
               * The network is too slow. One or more images are dropped from each triplet.
                 Try restarting stereo_view, increasing parameter 'queue_size' (currently 5)
[ WARN] [1297946077.740713539]: [stereo_view] Low number of synchronized left/right/disparity triplets received.
Left images received:      147 (topic '/stereo/left/image_rect')
Right images received:     155 (topic '/stereo/right/image_rect')
Disparity images received: 29 (topic '/stereo/disparity')
Synchronized triplets: 0
Possible issues:
               * stereo_image_proc is not running.
                 Does `rosnode info /stereo_view_1297946016943512050` show any connections?
               * The cameras are not synchronized.
                 Try restarting stereo_view with parameter _approximate_sync:=True
               * The network is too slow. One or more images are dropped from each triplet.
                 Try restarting stereo_view, increasing parameter 'queue_size' (currently 5)