If you are using stops/limits and not already handling the 5-cent thing, please try cent_set() at https://www.quantopian.com/posts/robin-hood-extreme-vetting#583e5c329d09850015000edb or one of the other approaches to it and let us know if that solves it.
On Robinhood I have both Cancelled and Rejected orders. I do not think those are the reason (in full) that at 2 months the paper trading version for comparison is up 137% while real money is 43% of that. Rejected orders say 'Not enough buying power', expected, would be because I used up cash in a loop already. Cancelled orders are my own actions at the end of the day to prevent the logging of unfilled orders while under development, none during the day, at least not lately.