Seems that sometimes free margin "Go freeze". Why? For example i use 15 BTC margin , make a floating profit of 2 BTC and free margin remain about 60.000 BIT 
In my experience at SimpleFX, fluctuation in an open position's profit/loss can decrease the amount of margin available to other positions but not increase it, not until the position is closed.
For instance, in your example, if your open position started to have a loss, that would make your free margin become less than 60.000 BIT, but there would be no difference in free margin if it zoomed up to a 3 Btc profit instead of a 2 Btc profit. Of course, once you eventually closed the position, then profits would be available.
I have encountered some sites which work differently, which would explain why it seemed off to you, but the above seems to always be how SimpleFX works, AFAIK.
