Many traders see a strategy perform well on demo and then struggle live. The difference is rarely mysterious.
Execution
Demo servers often fill orders instantly at the requested price. Live orders interact with real liquidity, so fills can be slower or at a worse price, particularly on fast moves and during news.
Spreads and costs
Demo spreads may differ from live ones, and commissions may be modelled differently. Strategies with small targets feel this most.
Account conditions
Leverage limits, minimum stop distances, swap rates and stop-out levels vary by broker and jurisdiction and can change over time.
People
Even with an EA, people switch systems off after losses, change settings mid-drawdown or add risk after wins. A written plan for when to intervene — and when not to — matters as much as the code.