Keep using those handful of replies to reiterate your pitch and value propositioning. It takes time and effort.
That said, I work for a company named Proxycurl that primarily grew to its initial major milestone of $100K through SEO and cold emailing. This article we published here about it might help out with your cold emailing.
If you're sending 60 emails and not getting a single open I have to first lean on the side of you're not getting good data / emails.
I work at Proxycurl, an Apollo competitor, and would suggest giving us a shot.
That said, here's an article that breaks down how to use our API to generate cold email lists.