As I know the MT Terminal can use one thread for orders executions. It means that while one order is on processing, the terminal cannot execute other operation. They Called that "Trade context busy".
The EAs from FSB Pro and EA Studio have a mechanism to check if the Trade Context is free or not. If the Trade context is busy, the EAs wait for it to be released. if it doesn't happen the EAs show a message.
A possible reason os downtime of the trading server or network delays. It is also possible to have too many experts that want to trade simultaneously, but the EAs may manage it.
Whatever the reason for that to be, it is out of our control.