There are many reasons you are/may be being rejected:
1.You have broken the rules before.
2You are not qualified for that group.
3.They are looking for specific people.
4.You are asking multiple times after you have been rejected.
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Qualifications
Administrator:Been on for longer then 6 months,loyal member,more then 200 posts,Never Broken Rules
Moderator:Been on for longer then 2 months,loyal,more then 100 posts,Never Broken Rules
Helper:Been on for longer then 4 months,more then 100 HELPFUL posts,specially trained,Never Broken Rules
Technician:Been on for more then 5 months,specially trained,in contact with Admin at all times,Never Broken Rules
Ban Specialists: Been on for more then 2 months,Read rules and Terms and conditions all the time,specially trained,Never Broken Rules
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NOTE:You dont have to do everything for the specified group to join.
NOTE:Even if you are fully qualified it does not mean you are part of the group.
Admin Chooses all groups.