I may have missed something, but I think there might be a problem with
deltarpms in all the current Fedora releases.
Looking at http://mirrors.kernel.org/fedora/updates/27/x86_64/drpms
there are eight available deltarpms for F27, and if you go to F26,
there are two.
At a guess, new deltarpms are being generated for each push, but the
old ones aren't being copied in anymore.