So many aspects why a user didn't accept in any campaigns;
Campaign managers see your posts as spammy, not constructive (together with your grammar),
Campaign you want to join is already full,
Your negative feedback on your profile might be one of the reasons also,
You didn't follow the rules of the campaign, especially when applying the campaign,
etc...
Mostly i do try to follow each possible instruction given by the campaign manager and also i stay far away from soammy posts. Also my Grammer is not that good, but yes it is acceptable.
Thanks for your suggestion. I really appreciate any kind of suggestions for now, as i will do try to improve myself in order to get accepted and to serve any signature campaigns.