There are a few reasons why this could be, but first of all, make sure that you are looking at the correct community, and that you have sorted the posts by "new". By default, posts are sorted by "hot", which prioritizes posts with higher scores, so a brand new post would not appear there. It's also possible that your post got caught in a spam filter or was removed by a moderator of that subreddit. Reddit also has a spam filter designed to detect spam posts and automatically remove them. However, legitimate posts are often caught by the filter. If a few minutes go by and your post isn't showing up in the "new" sort of the community where you posted, it has probably been caught by the filter. This is most likely to occur if you are posting to a community that you have not participated in before. Each community has an independent filter, for example, r/help's filter doesn't talk to r/pic's filter.
In order to get your post out of the filter, you'll need to message the community moderators and ask them to check the filter for you. On the desktop site, this link can be found in the right side community menu, and you can also manually compose a message to #communityname. Eventually, the filter will "learn" that your posts don't need to be removed.