each of my biggest two ballances have 17.871% probability.
that mean, that should be chosen to be minted in average approximatedly every 5.6 days.
That's certainly not how it works. You can use
http://peercoin.github.io/peercoin-POSCalculator/ for example to calculate percentages properly.
so I think, someone don't want to reward me or others to have big and frequent reward, but only small reward.
The entire network is trying to do the exact same thing you are doing at the moment: generate blocks to get the block reward.
Read: You're competing against a volume of ~100 million coins worth of valid transactions each day. Transactions that have been actively trying to stake for months on end.
And there's only about 150-170 blocks that can be generated on any given day.
Before you change that POS logic in that one specific update for wallet, reward was no problem.
Can you point out which exact update you are talking about? Because the PoS algo hasn't been touched in years.
To summarise:
Either combine your transactions and turn off stake splitting if you can't deal with sporadic-every-now-and-then rewards, be more patient, or get more coins to stake for you.