Multidimension array problem

Hi,
Trying to set up a multi dimensional array to act as a circular buffer.
Will copy 8 databytes into the array and increment a write_counter.
Then in different module when it comes around to service the buffer it processes each entry until the read_counter == the write counter.
My code is below;

PRL.c code

#define CMD_RXBUFFER_SIZE 10 //how many entries in my buffer
#define CMD_RXBUFFER_WIDTH 8 //ie 8 bytes wide
.
.
unsigned char cmd_rxbuffer[CMD_RXBUFFER_SIZE][CMD_RXBUFFER_WIDTH];	//ie from 0-->49 deep, 8 wide
unsigned char rxbuff_write = 0;
unsigned char rxbuff_read = 0;
.
.
//then in a function:
rxbuff_write = rxbuff_write + 1; //add 1 to current write counter
if (rxbuff_write==CMD_RXBUFFER_SIZE) //dont try to allocate data to a non existent array entry
   {
   rxbuff_write=0;
   }

memcpy(&(cmd_rxbuffer[rxbuff_write][0]),&DLL_Rx_Buffer.databytes[1],7); //copy the received data to the next available buffer

then in my next file TRL.c i want to read out the data from the new buffer

extern unsigned char cmd_rxbuffer[CMD_RXBUFFER_SIZE][CMD_RXBUFFER_WIDTH];
extern unsigned char rxbuff_read;
extern unsigned char rxbuff_write;

unsigned char TestData;
.
.
.
//then in a function:
while (rxbuff_read != rxbuff_write)
 {
 rxbuff_read = rxbuff_read + 1;
 if (rxbuff_read == CMD_RXBUFFER_SIZE)		{rxbuff_read=0;}

 TestData = cmd_rxbuffer[rxbuff_read][0];

TestData always equals 0.

NOW the CONFUSING BIT!
if I was to replace the rxbuff_write and rxbuff_read with a numerical value (eg 3) then it works, and TestData equals my data.
if i assign

rxbuff_write=3;

then it also doesn't work.
I have outputted the values of the read and write variables at each stage and they are equal to what I expect them to be, and loop around when I expect. And since I can manually assign the number into the first array index, eg:

memcpy(&(cmd_rxbuffer[3][0]),&DLL_Rx_Buffer.databytes[1],7);

THIS WORKS!
but:

memcpy(&(cmd_rxbuffer[rxbuff_write][0]),&DLL_Rx_Buffer.databytes[1],7);

DOES NOT???

Im totally confused why this wouldn't work. Does anyone have any idea?

Thanks for any help in advance,
Kind Regards,

Peter