Hi,
I have similar problem when I tried to read Freq and Power level. I discovered that you have to make sure that the first send/receive is completed then you can do the second send/receive. It has something to do with synchronization. To do that you have to creat a vi that tell the previous task is completed and ready for the next task. This can be done by just setting up a simple respond message . It will look like this: Send --- wait for resp message---message receive . Now you can put the whole thing in the loop. If this is not help, I think you can put those steps into sequence (it's only a couple of them).