Communication problems to an Arduino Board

I am working on MATLAB to communicate with an Arduino. The code is very simple. I am sending an A to the Board, and want to read 100 bytes from the device.

I keep getting this Warning: Warning: Unsuccessful read: The specified amount of data was not returned within the Timeout period..

and only 47 Bytes are read, which is the header, meaning that sending "A" to my board, did not have any effect on it, is there any solution?

arduino=serial('COM8','BaudRate',115200);
fopen(arduino);
fwrite(arduino, 'A');
daten = fread(arduino, 100, 'uint8');
fclose(arduino);