I have an expert that uses a single chart to trade multiple symbols but I need to find a way to improve performance

I use an amazing developer but I need help from someone who might have an idea on how to improve performance on multi symbol trading from single chart.  Currently if I backtest my set files individually I get results ranging from 80% + wins but when we backtest with the expert that does the multi symbol trading from a single chart we get 50% wins. I understand backtesting isn't always accurate but in my mind if each individual set files tests a certain way shouldn't they test similar when we run the multi symbol expert? Ultimately I need to find a way to make the multi symbol expert behave like the program that runs them individually. I am not sure if I am explaining right but maybe there is a better way to run the multi symbol program so it behaves similar to my expert when running a single chart. I love my developer but I want to solve this issue

The multi symbol EA uses the same code as the single EA but just on multiple symbols on the backend. I have heard there could be problems with getting the right price when using one chart for multiple symbols. Is there a fix for this?