Getting Firmata to control WS2801 LEDs
It's possible that time.sleep is causing a latching issue. You should instead use pymata-aio's sleep function:

uno.sleep(0.001)


The documentation for it can be found here: http://htmlpreview.github.io/?https://github.com/MrYsLab/pymata-aio/blob/master/documentation/html/pymata_aio.html#module-pymata_aio.pymata3

You could also try a different pin and see if you still have the issue?