Remote facilitation is a new practice, therefore it needs to be improved with every session. Design an easy mechanism for participants to share oral and/or written feedback (give them choices that fit different personalities and also different abilities). And ask them for consistent feedback; thumbs up are great, but you can learn more from a paragraph.
Because the sessions are getting shorter, the time spent in between is getting longer. And that's a good thing, because it provides more time for the incubation stage. Tell the participants about the importance of this stage in creative thinking processes — let them know [how] they can get back to the virtual whiteboard in their free time, and encourage them to add all ideas that spur offline.
The engagement and momentum you build online is at risk of dissipating once the session is over — this is why every session should end with a very clear next step (whether it's homework, individual or group activities). Tell the participants what they should do next, and put it in writing as well. Schedule one on one discussions for further clarification, if neccessary.