Hi,
It looks like SEP client is taking Full.zip definitions from SEP manager instead of delta
Do let me know the following -
1. content revisions kept in SEPM
2. last definition dates on affected SEP clients
In any case you need to keep no. of revisions in SEPM for 30 days (recommended) so that SEPM could generate the delta definitions and clients could receive it.
Full.zip definitions are usually large in size, this may be reason why some of the SEP clients are not receiving these definitions due to bandwitdh and size