I dont know how but giving the route absurd amount of transport ships changes the colour from green to white after 1day the amount of transports needed will automatically drop (need to set it back to auto assign number of convoys after you manually make the convoy) this somehow fixes it thankfully my troops didnt starve to death )The usual question:
Is there an ally who supplies your troops?
And this ally perhaps with not enough supplies? If so, give him enough stuff.
Otherwise, try deleting all convoys, set it all to automatic, next midnight tick the routes will be realigned and remade. Sometimes this helps.
[...] changes the colour from green to white after 1day
# Max provincial energy/metal/rare materials depot size.
5000 #9999
# Max provincial oil/supplies depot size.
15000 #9999
# Desired provincial stockpiles for oil and supplies (needed to supply friendly units in that area for X days). Used by auto-convoying.
15 #7