As you might know that the rewards of your posts decrease as time passes by.First I doubt but now I'm 100% sure it is happening with me and some other users also confirmed the same happening to them.
By researching on this matter someone gave me a logical explaination.
See the rewards to be given to total steem community is fixed.This being said if some authors better than you post they get more reward so they take your share.It's like everyone is competing for one piece of cake.
This should be kept in mind that I'm not sure that this is the case.There might be some other explaination to this.
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From @mocool1