Email Open Rate Moving from Keap to HubSpot

A client moved from Keap to HubSpot a few months ago. Since then our monthly newsletter email - over 17000 names - dropped from avg 10-15% to 3% open rate. Our weekly email newsletter dropped from 25% open to 10% open rate.

Any ideas why or what we should be doing differently? It appears from the HubSpot data over 50% of our emails are Gmail, and they now have a 1% open rate.