multiple calls to DAQmxWriteDigitalU32

I need to write digital lines with a long initial pattern, followed by faster pattern. To accomplish this, I  first set sample clock timing to slow (1 Hz), and write data using DAQmxWriteDigitalU32. Next I change the sample clock timing faster (500 kHz), and write again DAQmxWriteDigitalU32. The problem is that only the first DAQmxWriteDigitalU32 writes data. The secondDAQmxWriteDigitalU32 does not write any data. What could be wrong  here?

    # DAQmx Configure Code
    DAQmxCreateTask("taskSrlO",  byref(taskHandleSrlO))
    # DAQmx Create Channel
    DAQmxCreateDOChan(taskHandleSrlO,
            "Dev1/port0/line0:1, Dev1/port0/line2:7",
                    "", DAQmx_Val_ChanForAllLines)
    # DAQmx Timing Setup
    DAQmxCfgSampClkTiming(taskHandleSrlO, "", 1,
                            DAQmx_Val_Rising, DAQmx_Val_FiniteSamps,
                            1+dataSrlO2.size)

    wrtSrlO = c_int(0)
    # DAQmx Write Code
    DAQmxWriteDigitalU32(taskHandleSrlO, 1+dataSrlO2.size, 0, 10.0,
                            DAQmx_Val_GroupByChannel,
                            dataSrlO2, byref(wrtSrlO), None)

    # DAQmx Start Code
    DAQmxStartTask(taskHandleSrlO)


    DAQmxWaitUntilTaskDone( taskHandleSrlO, 10);   
    print("Written1: %d\n" % wrtSrlO.value)

    # DAQmx Timing Setup changing sample frequency
    DAQmxCfgSampClkTiming(taskHandleSrlO, "", 500000.0,
                            DAQmx_Val_Rising, DAQmx_Val_FiniteSamps,
                            1+dataSrlO2.size)
    # DAQmx Write Code
    DAQmxWriteDigitalU32(taskHandleSrlO, 1+dataSrlO2.size, 0, 10.0,
                            DAQmx_Val_GroupByChannel,
                            dataSrlO2, byref(wrtSrlO), None)

    DAQmxWaitUntilTaskDone( taskHandleSrlO, 10e-3);   
    
    # Serial Out End   #####################

    print("Written2: %d\n" % wrtSrlO.value)