Rule #1 of RS-485 is that no two manufacturers designate Data A and Data B the same, so always try swapping data conductors.
The USB device probably allows the software application to command Transmit and Receive modes directly. The RS-232 / RS-485 converter requires handshaking or timing settings to switch from Transmit to Receive.
Do you have RTS and CTS connected on the RS-232 side, or just Rx, Tx, and GND ?