First of all, make sure your post has been published to the board. Posts published to personal profiles are not shown in the public feed and can only be seen on your profile page.
Then check that you are looking at the correct board, and that you have sorted the posts by New. By default, posts are sorted by Hot, which prioritizes posts with higher rating, so a brand new post would not appear there. If you are looking the Home feed, then make sure you are joined the board where the post was published.
It is also possible that your post was removed from the board by the board moderators. If so, check your notifications. Posts removed from the boards also disappear from the feed. In general, the moderators of the board specify the reason for the removal. If you think that your post was removed by accident, you can ask the board moderator about the reason by mentioning it in the comment using the "@" symbol.
Also, your post could be deleted by platform administrators if it violated the platform rules.