I hear this often, and yes it can be very frustrating to get rejections. I've done 200+ informational interviews over my career, and probably gotten turned down by 2000 requests. I wrote a post about this problem specifically a while back: https://grahamwahlberg.com/2016/03/22/informational-intervie...

But summary is:

1) Are you the people you are reaching out to “warm contacts”? By that I mean is there someone you know who knows them? If not you may want to try to start with people you already know.

2) What level of experience are you targeting for coffees? 1-2 yrs experience engineers? 5-10? 30-40 years? I have found that targeting people your own age yields worse results, but people just a little ahead of you in their career are more likely.

3) What kind of people? Some people are intimidated by going to coffee with someone new and some aren’t. If you can (and this is tricky) figure out who is more likely to say Yes.