My fin won't slide back

Hi im brand new to SUP. I just bought a board and fin off of someone and I am trying to put the fin on the board. The board is a Tahoe Bliss and the fin is also a Tahoe fin. I can get the back part of the fin into the square wide opening but it will not slide back either direction into the channel. However if i turn the fin around so that its facing the wrong direction then it will slide either way into the channel. Obviously that won’t work! I notice on the fin that the screw is much longer on one end than the other. Is this normal? Am i doing something wrong? Did i buy a defective board or a defective fin? Here is a picture of the back of the fin metal part thing:

Appreciate any help!

Try the SUP Board


Gently tap it so it is centered …
Lay on table / work bench so its not going to be moving around >>> with the pin just hanging off the edge … then just tap with hammer … A plastic one if you have it but no worries really.