That’s known as dilution. Delegates only forge a set amount of Ark a day (About 422). As the delegate gets more votes, the more that set number of coins has to be spread out.
The formula is as follows for an estimate of your daily share:
422 x (yourWalletSize/delegatePoolSize)
As you can see, as delegatePoolSize increases, the resulting number decreases. Right now, biz_classic is the #1 ranked delegate with over 2 million votes. Our delegatePoolSize is over 2M, the largest of any delegate. We were not ranked #1 back in June or July. Now we are, which explains your diminishing returns.
Your September 5th payout was also affected by a network-wide outage a few days ago where EVERY delegate missed out on over 200 ark. That outage affected us so we paid out less because we forged less Ark. Check the /r/ArkDelegates subreddit to see all the posts about reduced payouts from various delegates due to that outage.
If you want to maximize your payouts, you should vote for a delegate ranked lower with less dilution. Note that a lower ranked delegate is also at higher risk of dropping out of the top-51 and not be able to forge anymore. Our large pool size is what makes us the most stable delegate (See: https://classicdelegate.biz/stability). Voters can “set it and forget it” without worrying about us ever dropping out and forcing them to switch to continue to get paid.
Please read the OP carefully. We’ve linked instructions on how to guarantee an automatic refund. Your transactions got forged by other delegates which means they collected your fee so we don’t have it to refund back to you.