GUP server is downloading delta packages but not the full.zip file

Hi,

In SEPM, for 2 groups, the GUP server is downloading only delta packages but not the full.zip file. The GUP server is updated with the latest definitions but the clients are still out-of-date for one month even though the server has enough bandwidth size and 20+ GB free disk space. One GUP has 12.1.2 version and other has 12.1.6 MP5. May i know why it is not downloading? The server is communicating with SEPM and there is green light too and with latest policy serial number. I still could not find the reason why the full.zip is not downloading.