Downloads are fulls and no deltas, network bog

We have a new SEPM 12.1.5 server, and have some remote locations updating to 12.1.5 via GUPs. We've received calls of slow network performance. So I've made changes to "throttle" the bandwidth using the steps in these forums. I decided to go ahead and install the Content Distribution Monitor and found out our network is being bogged down by Full downloads.

Why is it doing that, and why aren't any Deltas going out?

Thanks