France 14, Race to the sea and forts

[stigmata]

Long time player who has gotten back into the series after being away for 4-5 years. I am playing an email game using the Race to the Sea scenario and loving the heck out of it. I find the WWI games to be my favorite. But I have no clue how forts are working as far as supply and isolation goes. I am besieging Antwerp and have a number of forts surrounded. They are tough to crack as they do not seem to get isolated or run low on supply. However, after some days I just noticed that all of the troops in one fort are now isolated. Question is how does it work to gain isolation? Is is just a matter of surrounding them and waiting a few days, or something like distance to HQ? Or is it a random event? I just don't know. Got one isolated and ready to assault but am not sure how I did it. Looked through the manual but can't find anything.