I have been writing a lot about being effective on email and I wanted to share another quick strategy that I have found really effective to get someone to respond to an email after they go cold: tell them you will call them later that day.
I will often follow up with them with a brief reference to the context of the original conversation, for example:
I just wanted to circle back with you on [enter context of email]. Please let me know if you have any questions or need more information from my end to move things forward.
I will give you a call later today to reconnect and discuss.
This helps create a sense of urgency for them to repsond. I have found they will either call you right away or drop you an email with next steps. It can be very effective.