Video call only works in one way

When server A’s SIP user called to another server B’s SIP user, they could talk and see each other’s video after clicking ‘Start Video’ on X-Lite.

However, when server B’s SIP user called to server A’s SIP user, they could talk but could not see the video even they had clicked the ‘Start Video’ on X-Lite.
The X-Lite was sending the video but no video was seen on each side.

The SIP debug message appeared normally and it could detect the audio and video format.

Note: server A is Asterisk and server B is Asterisk

Could there be any version incompatibility problem?
Please advice us.
Thanks in advance.

The problem has been solved. It was actually due to NAT issue.
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