Then your problem might also be the duty cycle of the gateway. TTN Console will nicely tell you when it cannot find a gateway that can handle the downlink. (Which, for example, also happens when many nodes try to do OTAA at about the same time.) I don’t know what your server does in that case. (This is one of the reasons TTN’s fair access policy allows for only a few downlinks per day.)
Also beware that current gateways are half-duplex. When transmitting a downlink they cannot receive any uplinks. (Yet another reason for TTN’s fair access policy to limit the downlinks.)
…but TTN’s network server is waiting until the very last moment, allowing for a little network delay.