What exactly does the message say regarding you not entering a competition?
The reasons I am aware of are...
If your disconnection rate is below 70% you won't be permitted to enter any of them.
Some of them have restrictions on your division etc.
You can only enter after the start time and as long as there is enough time left for the competition opening (i.e you cannot enter with only 10 minutes to go).
Finally sometimes it can be port restrictions. For example if you cannot enter a lobby and only be able to play quick matches (The way i got around was by enabling uPnP).