You do not have any more free margin. That could be the reason.
Check does it says to you that the margin level is close to 100%.
If yes, then you have open to large trades and your margin does not allow it to open more.
It can also happen that you have losing trades and free margin is spent on those trades.
If that is the case you need to reverse losing i to wining trades and when your free margin increases ypu will have space for more orders.
Attach the image of your terminal with account balance and margins so I can see what is the current status.