In POS failed mintings are normal, one way to decrease the number that fail , is to use coin control
and combine the smaller amounts into a larger amount.
If you look at your chart everything that was over 13 interest minted, and most everything with under 1 interest failed to mint. We called it Dust when the amount are too small to mint.
So combining the dust (smaller amounts) together for minting will increase the chance of minting.
Fyi: Coin age does reset when you combine the smaller amounts.

dmd pos have a feature called group stake
coins with age abover 30 days and combined size below 100 dmd
try to mint together and if successful they merged together
manual use of coin control is not required
dmd have successful anti dust mechanism
but as the word pos
atempt say they can fail
thats normal behavior and they will try again
What size amounts are Group staked together, because his interest attempts show a very small amount being minted. Which makes it look like the group stake is not working for him.