Sorry you are finding supply frustrating, if you understand the basics it is quite simple really.
Have a read of this article
https://www.theblitz.club/message_boards...?tid=70586 which explains how the supply rules have evolved over the series and should help you understand the feature.
In its most basic form the supply rules require every unit to be able to trace a supply line free of enemy units or their ZOC back to a friendly supply source (or any map edge if supply sources are not included in the scenario), when judging this terrain like rivers has to be taken into account as supply cannot be traced across apart from at undamaged bridges or bridges laid by engineers.
Not knowing which supply rules you are using (default, virtual supply trucks or explicit supply) I am guessing that if you have indeed put a continuous ring of units/ZOC round your opponents forces and they have not been isolated then they are drawing supply from a supply source located on the map rather than on the map edge, (players sometimes assume all supply sources are on the map edges) or there is simply a gap in your ring you have not noticed (done that myself).
Without more info that is the best answer I can give you.
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