Fighting spam has always been a tricky thing. Spammers keep getting smart day by day and continue to hammer your site no matter what tools you use. For a long time, Akismet used to be effective and the only tool one needed to fight spam. But in recent times, its not enough. And then there’s a problem of false positives which personally started increasing here.
If you are looking for a better spam plugin, I would personally recommend you to try out AntiSpam Bee. I have not received even a single zero positive in past few months of using it. You don’t even need to run Akismet in parallel to it. Akismet where learns and checks spam against its server rules, AntiSpam Bee uses various methods to detect spam some of which are CSS hack check, IP validation, checking against previously received spam, checking against a DNSBL list, language of the comment and several other factors.
Give it a try and let me know if it worked for you like it works for me.