Situation in image https://imgur.com/a/KBYMZYC
What are the conditions for buffer depots to try to get resources? Shouldn't they work like buffer chests and try to get resources all the time. Here all supply depots on left are full, but this buffer depot does not pick up anything.

Is there enough in the supply depots? It might be that its all consumed by normal request depots

well all supply depots are full, but it might be true that it may all be consumed already be incoming drones (it is a long road)
I will add more supply depots

as far as i could see, the depot readers read the exact real value how much is in the depot, but the network reader shows the amount of available(!) items, meaning it shows the value after subtracting any pending deliveries. on my oil route, the value for oil on the network reader immediately dropped by 500 for each drone when it was sent, and as soon as the drone had taken the load, i could see the fluid depot slowly adding oil again up to its capacity.

to check your setup, you might connect all supplier depots to a pole by green wire and a network reader by red wire. then you should be able to see both values at the same time. or use nixie tubes or the magic lamp instead of the pole.

Yeah it's just as you say. I did not know bots reserve resources once they go out. This makes a lot of sense. Thanks for info