Downlink messages logged by Chirpstack, but do not have any effect

Dear Community members,

My end nodes are configured as Class C devices.
I have a python application dedicated for managing uplink and sending downlink messages. I am properly receiving all the uplink messages, but when I try to send some downlink messages, Chirpstack actually logs that activity, but it only has some effect on end nodes if it is being sent shortly after receiving an uplink message (class A device characterics). In the attachment, I am sending You a screenshot of Chirpstack logs and here You can that when a downlink message is actually being sent to the end node, I get a response on that same port. For example, here I am sending some commands on port 4 and at 9:18:45 I receive an answer on that same port. As You can see in the screenshot, I have tried to send downlinks before, but I did not have any effect on my end node because it has not been sent almost immediately after receiving an uplink.

What end device are you using? Any error in the Chirpstack logs?

What LoRaWAN library are you using?