My own post disappear or are very late

I am working with Gnus as news client.

Often the messages I post in a newsgroup do not appear for me in the
newsgroup or after a delay of several days. It is not consistent.
Sometimes I see a message immediately, sometimes after a few days,
sometimes never. I have the impression that the message with which I
start a thread has more often a problem as others, but I am not sure
about that.

What could be the problem?

-- 
Cecil Westerhof