I had the same experience in April this year. I was working on a strategy for 15 months and had it running in in simulation from Jan-March with the same results ... All automatically. So I started in April and everything changed. I had 100% loosing days in April ... No target were hidden, entries were stopped quickly.
But I kept trading this mech system through June and surprise, since July everything runs like in the development and simulation phase and like expected and wished .... I figured out that the market behavior was absolute different from the first trading session in April till last trading session in June. Sizes of trades, bid/ask etc was absolute different. Moves were totally different. For me there was a new player/algo in the market for just 1 quarter ... I always see huge changes when a quarter begins and a quarter ends ...
You said you were successful in September ... From October on it went wrong ... So may because of the new quarter and someone figured out the same opportunities like you did or saw that someone was playing like you did and thought it is time to go against you ...
All seen in the entire trader life ...