Are you breaking fin screws, or footstrap screws?
Since you mention "screwing a screw into the board" I'll take a guess that you are breaking the footstrap screws.
Are you using a power tool to drive them in, or the Tiki tool (or other #3 Phillips screwdriver?
Make sure the screws are 6 mm x 28 mm long PT screws (the stock FS screws provided with all Starboards).
If the screws were too long, then they would bottom out.
Also make sure you are using all the correct "stack up" of washers and K9 locks so the thickness of the "assembly" is correct, or the screw could also bottom out.
I suggest you get a bar of bath tub soap (the kind you use to take a bath) and "lube" the screws with soap before installing them.
They will go in much easier, won't loosen up, and you can get them a little tighter as well.
Perhaps you are just tightening them too much. Maybe soap the screws and just snug them up, then go back after a couple of sessions and retighten them again.
Hope this helps,