Since there has recently been a plethora of threads started on this issue, I believe it is time for a sticky thread on the topic.
99.9% of all ranking issues can be solved by looking at the Ranking Info page of a report. This can be accessed from the main drop down menu, at the bottom.
As an example report I will be using this report for now.
The first thing to look at is the Fights Info section, which shows any sort of meta problems with the report, for example if it has been flagged by an admin to not be ranked, or the date it was last re-exported to the rankings. Looking at the Ranking Info page you can see that one of the hunters in this guild has been using an exploit to tame pets and has caused a Forbidden Buff flag to be placed on their report, making everyone's rankings invalid.
The next section is the Exported Fights table. This table shows a summary of all the fights in the report, showing how much damage was done and the minimum required to rank. Minimum fight duration is also shown here, although this is set to 20 seconds for most fights so your problem here, if any, is usually not enough damage done. This can happen by starting the log late, or having your logger DC in the middle of the fight. Another possible reason that will pop up occasionally as gear gets better or fights are nerfed will be new ways of killing the boss come up that do less total damage. If you think you are running into this issue, please open a thread on the forums and explain what is going on and we will take a look at it.
Moving down to the fight details you can see the DPS / HPS numbers used for the rankings, and what percentage they are of the #200 spot for that spec. Remember that rankings are based on DPS(e) and not DPS.
If we can't automatically determine your spec it will be shown as Unknown, and no ranking will be available. This is one of the few problems that should be reported, and we will look into it to see if a change to the weighting is in order to help with determining specs.
You can also see on this page if you already have a ranking for that fight in a different report that is higher than your ranking in this report. You can only be ranked once for a fight, so the higher one will take precedence. If you look at Lady Deathwhisper you can see that 3 players from this report have a higher ranking in a different report for that fight and thus are not ranked in this report.
The important columns for checking whether or not you should have gotten a ranking is the "% of limit" and "Rank Limit" columns. The Rank Limit column shows how much DPS / HPS the #200 ranking spot did, and the "% of limit" shows where your value was in relation to this. A "% of limit" that is under 100% means you did not do well enough to be ranked, while a number over 100% means you will be ranked.
Hopefully this post will show you how to check your own reports as to why they have not been ranked, and show you where the problem is. If your ranking problem is something this page is not showing, or you think that it is incorrect and can back up that statement, then please post a topic on the forums describing your issue.