Most people are very busy. It helps to remind yourself in these situations that you don't matter (and I mean that respectfully and empathetically). Often it will take my clients 1-2 weeks to come back to me even though they say "they need my services right away".
Steli from close.io talks about making SaaS cold calls/emails for 5-7 times until he gets a positive reaction, unless told to F.O. People are busy.
P.S. The startup is pretty big now as it has millions worth of funding.