lost packet or lost

i have a embedded plateform who sending packet of 28 bytes.
(19200, 8,NONE,NONE)

My soft Qt who received data:

1) serial port config, set baudrate, setBaudRate
2) connect
3) open()

in readdata() without any code with Qdebug()
2 frames is correctly send,but One frame is not correctly. (not have 28bytes)
i miss data of trame at the begin.

if i add clear() after opening port, most byte is incorrect,(not have my frame)

it's the same problem is terminal programme example Qt!!!

the data is correctly received may be after.