When you say "having trouble getting the board to go downwind", what exactly is the problem.
Does the board slow down...... this could be because you haven't sheeted out and your sail is stalled..... or perhaps because you don't have enough wind speed to go very deep off the wind.
Are you tipping the rig slightly forward (let the end of the boom come up slightly).
If you keep the rig fully sheeted in and the foot locked down on the deck (near the F/A centerline of your board) you will not be able to generate any "off the wind" pressure to help you turn your board off the wind.
Where do you run your mast foot.
If you have the mast foot really far back, to minimize the wetted surface and give you the best speed and ride, the CE of your sail could be getting back behind the fin a bit and making it nearly impossible to drive the board off the wind without bringing the boom end up some to get the CE forward of the resistance of the fin.
Are you "unweighting" your heels slighty and trying to push down with your toes (rocking your foot to a more "toe pressure" vs "heel pressure" attitude) to help your board turn off the wind (sort of like initiating a jibe).
Also, if you have lots of rear foot pressure (across the top of the board) this will make the board want to head more upwind all the time.
Lighten up on the rear foot pressure (across the top of the board) and try to put a little downwind pressure (vs pulling up away from the water and upwind, to go upwind) on the front foot and footstrap.
Letting the rig come up a little (boom end up very slightly) and putting more pressure on the front foot (across the board and downwind) will feel is little like you are
"initiating" a catpault, but if you do it carefully, you will turn slowly off the wind.
As you turn off the wind you also have to sheet out some to keep your rig from stalling.
And, at some point you will get so far off the wind that you lose all benefit of the apparent wind, and the board will slow down. (It's wide, but not as wide as a formula board and you probably aren't running 10 m2 + rigs with adjustable outhauls that make sailing formula boards reallyfast, and very deep, even possible at all.
Hope this helps,