You can control your speed by turning the board uphill (or close) both on front and backsides.
As you said that front sides seemingly work out well, most probably you do not push your heelside hard enough, so you gain too much speed, so much, that you end up not even being able to do the next frontside, instead you skid/brake. Right?
Try to work on your heelsides, initiate the turns more agressively, push harder and then jump into the next frontside. There are many good drills on this website.
Look at some of the EC videos, you'll see a perfect speed control (let's not go into the 'style' conversation now....) Speed control is paramount especially on black runs. On blues you'll survive even if you are not able to do it, but there is no way you come down a black run carving side to side and no skidding if you are unable to control your speed by turning properly.
You can check your progression by looking at your trenches from the lift (if there is a place where you can do this) and you'll see the difference between your front and backsides.
You could also try a better board, but most probably it is not the board this time. You can do nice turns on a F2 SP although it is far from being a great board.
Kindest