Single Fin wont fit in box forward but it fits backwards? What does that mean?

Ive tried sanding the sides the bottom and nothing seems to work. Im starting to think someone put the fin box in backwards. Is that possible or no? Because If I put my fin in backwards it fits snug like a glove but when I put it in forwards It just wont go. I also have two tipes of fins and both of them same problem. Any help would be greatly appreciate because this is pissing me off. Thanks

What kind of box is it? Single fin, futures, fcs?

Single Fin. I actually played with it some more and It fits fine now if its all the way back but it cant go all the way forward. Better than nothing I guess but something is definitely wrong.

Which side of the fin is the screw on the front or back?

If the screw is on the back then the distance the tab that holds the fin in the box slot to the bottom edge of the fin may be a bit long. You might be able to use a file and take a little of that down.

Measure the distance you have from the bottom of the box to the top and bottom of the slot on both sides and front and back. See if it’s the same all the way around. Then measure the distance from the bottom of the fin to the top and bottom of the tab that fits in the groove in the box.

If it’s the width of the fin, I use a file and take a little off the sides of the box until the fin fits.

I have that on one board. mine is caused by there being a gap around the plug at the end it fits. i tried adding a bit of resin and a few other things silicone etc but nothing works and it slowly soaks up water. but it surfs great so it will just be the james dean of surfboards