TMS320F28027: Simulink rate transition block issue

Hello, I have designed an echo program. When two bytes are received, they are transmitted (both via SCI). However, it only works when the two bytes are received one by one. If a two bytes vector is received, it is not sent. This is so because the Tx Subsystem block is not triggered. But if I cut the Tx Subsystem block and paste it in the Rx Subsystem block, the program works perfectly. So the solution would be to get rid of the rate transition blocks. But I do not like it because the interruption would have to manage it all. Any advice will be wellcome. Thank you.