What exactly does that mean? Are you seeing uplinks in the gateway’s Traffic page in TTN Console, but not in the application’s Data page?
If, instead, you’re trying to say that uplinks work regardless if your node is using Class A or Class C, but downlinks only work when the node uses Class C, then there’s probably a timing issue.
In Class A, if implemented well, the node should be listening for a possible downlink exactly 1 second (RX1) and 2 seconds (RX2) after it has transmitted an uplink. Apparently that fails.
For Class C, even though TTN does not support that yet, the node will be listening for a downlink all the time. This obviously needs a lot of energy, but apparently then the node receives the Class A downlinks that were transmitted in either RX1 or RX2.
(And please don’t ignore my remarks about the duty cycle limits.)