Supposed that I have a set of lenght study (50) and I ´m using a (Maximum Number of bars study will reference) (50) ... so lenght study is <= Maximun Number.. (ok) I don´t understand why the backtenting result change when i go up with (Maximum Number of bars study will reference) to (500 for example...

I´ve been creating a strategy automated trading and it has been wonderfull in backtesting but i´m not having succes in real tradings.
Anyone could help-me to understand better what kind of mistakes i´m doing and what is missing in my analyses?