Well what are you using for a projectile. One thing you should probably do is make the coils themselves a bit bigger (the look from the picture like they onyl have about 20 turns in them). Also since I'm assuming your projectiles are probably nails with the heads ground off, you might want to switch the nails themselves. Some nail steels aren't as magnetically active as others. Though that will come down to trail and error.
If you've got a few working coils though, and aren't too worried. What you can do, take 3 coils, and place them right next to eachother. Then a little down the line do another bank of 3, and farther down another 3. Set them all up with their own relay, but in a manner so each bank of 3 will trigger at the same time.
(With doing this, itll give you the power of 9 coils, but the complexity of 3) then it comes down to doing testfires and trying different timings. Probably will have to use micros in the timing really.
You might be helped severely by throwing together a chronograph (actually im gonna ask advice on the forums about how to make one here in a few mins) And testing the speed of the projectile at several steps of the firing.
Another experiment you might try, do 3 banks of coils. 1,2,2, coils. Hook it up so that on firing, first all the coils are triggered, each having its own cap to run. After passing coil 1, the second 2 banks are triggered again, drawing power off a second set of caps (also taking another few couple relays). As it passes the second bank, the last 2 coils trigger again, along with another set of caps. That means that at initial, the projectile will have the highest acceleration, and then as it moves through the barrel, gets sped up again by the new fresh charge from your second cap bank. Then right before it leaves the barrel, gets another last burst of significant power. This can only work though if you have measured out the speed at several points and can trigger the coils at the right time.
Also it lets you use all your coils. If the explanation isnt clear, later on ill draw a diagram. I tried to do this once with my own coilgun. But as it would happen, there was too much current going through the projectile, and it had a habit of melting the plastic I made the barrel out of.
(I was using it as a conductor to make the contact for the subsequent coils. To alleviate needing computer timing, note this was long before learning of arduino lulz) Yeah it was a dumb idea, but charging single coils at a time, it worked great.