Asterisk, no video with RTMP SIP Gateway and Xlite. Why?

It can be Asterisk issue.
Please check Asterisk video codecs in the sip.conf config.

Xlite supports H.263 only.
Asterisk may support both H.263 and H.264 codecs.
RSGW(RTMP SIP Gateway) supports both H.263 and H.264 and H.264 as priority codec by default.

For example, Asterisk configuration is:
ulaw
alaw
h264
h263

Here h264 codec has high priority.
So, according the Asterisk settings the media path might be:

Flash – [H.264] – Asterisk -[H.263] – Xlite
And here can be Asterisk issue if Asterisk does not perform H.264-H.263 transcoding properly.

To avoid the possible issue, please try to use exactly one video codec H.263 or H.264(preferrable) in the Asterisk settings.
Or if you know about a peer that the Asterisk peer is Xlite, you can set H.263 as priority codec for that peer.