Set:
kill percent : 100 <-- will not kill unless spam score reach 20
tag percent : no change if you are satisfy with the current tagging level.
I am assuming your know at least a few email did not come through, ask the sender to resend the same email and see if they come through this time. You properly have to check the junk folder in the web interface.
And out of curiosity, check the spam score using 'show original' in the web interface and let me know
Another solution is if those important but discarded emails come from a limited group of email addresses, add them to white list in user preferences.