On-Going Volunteer Training

We are now active in the municipality we are piloting our scene response program and have a 3 regular scene activation team lead volunteers and a few others who have volunteered to take a shift when the regular volunteers are unavailable. We have started with 2 12 hour shifts 7am-7pm and 7pm-7am. However, we have run into a few situations where these create too much unavailability from our regular volunteers and our partial volunteers have been unable to pick up the shifts. We are considering changing our shifts to the following: four 4-hour shifts from 7am-11pm and then an overnight shift of 11pm-7am. Would love any input for how you schedule your on-call scene activation team leads. Thanks for your help!