Followed up and ignored albeit good conversation

Hi,

I had a really good conversation with one of alumni - spoke for 40 minutes, and he even told me (without my solicit) that he would refer me to a few other analysts.

The next day, I follow-up and kindly ask for referrals - no reply and it has been a week.

Do you think he's not responding because 1) flooded with e-mails
2) didn't like me
3)sent out my e-mail too late? (should'v sent right away instead of waiting a day)

In this case, when should I follow up without coming off as overly aggressive?

Also, sometimes I go back and forth with alumni, they ask when it would be good to talk, I tell them my availability, and they simply do not respond. Is this because the times I provided did not work for them? Please let me know how I should go about setting up times for calls and following-up when people ignore.

Thanks