issue with RIOT and openMote-B

Hi , Recently I have purchased 10 openmote-B Board . i started working with the board i have searched services. contiki-Os don’t have the support for openmote-B but RIOT and openWSN have the support for openmote-B . i started working with RIOT-OS ,in RIOT github page TUTORIALS are there i have kickstarted there,I made the environment by installing the required installation ,then i flashed the TASK-01 to my openmote-B actually TASK-01 was running a SHELL in the board and printing the BOARD name ,the code flashing was successfully done and I got the Output also, but the problem was if flash any other TASK code or the same TASK code it was giving the following ERROR

ERROR: Timeout waiting for ACK/NACK after 'Synch (0x55 0x55)'

I’m unable to solve this issue i tried a lot please give suggestions to solve this issue