Great to see experimentation!
:)
Share the smallest version of code that you can so we are on the same page.
Would like to know the BAUD rate, delay between writes, etc.
Still SerialUSB?
There are Notes and Warnings here at the bottom of the page related to Serial:
I get the same results with an Arduino Uno R3 and Arduino Mega 2560 R3:
The above may not apply directly to SerialUSB and can’t test it.
I am sure you can! Serial communications can be a challenge at times.
Related:
Due - SerialUSB.write() issue - Programming Questions - Arduino Forum < some excellent suggestions in here!
Also take a look at Related Topics in topic above.
:)