This could be because the moderators of a subreddit are eliminating your posts, or because the spam filter is removing your posts. The most likely reason for this is that the moderators of the subreddit where you're commenting are deleting your posts.

If you've just published a new post in a community with a lot of upvoted posts, it's possible that you won't see it until you sort by new. Some forums have rules about how to format your post and what you should and shouldn't include.

Posts go live right away. Your comment has been deleted. Filters for low karma and/or new members may be in place in different communities, removing posts. The message from the community's moderators is required.