Can query serial port with NI MAX, but doesn't work in Labview

I'm trying to control a Coherent Sapphire 488 with labview. It's connected via a serial to USB converter into the computer. I can communicate with a device using NI MAX, but when I try to run the vi (attached), it works for several queries, but evetually returns a null character, after which I can no longer communicate with the port. If I then go into the NI MAX interface, every time I try to read it gives the "VISA: (Hex 0xBFFF0015) Timeout expired before operation completed" error message.There are also several queriess from the instrument's manual that don't seem to work at all (with or without MAX) and always return a null character. Any help is appreciated. I'm certainly a novice with device communication. Thanks.