When you open positions, remember that your total used margin must not exceed your free margin.
Here is an example:
Let's assume your account balance is €10,000, with €3,333 already in use. Thus, your free margin is €6,667.
You want to invest €700 in EUR/USD with a 30 multiplier/leverage. The margin you will need to open this trade is:
Margin=700*30*1/30=€700
Thus, your free margin, €6.667, is less than the margin requirement for this position, €700. This is why your trade won't be open, and you will see the Insufficient Funds warning on your screen.
To resolve this issue, you have to trade a smaller amount.