Checkers only move diagonally which explains why you can't capture A7. To capture a piece, you must jump over it to the space beyond it, which is why you can't capture F8 (it is on the edge of the board).
i have played a few of these now it seems that checkers get stuck at the top or bottom..if they cant get in the corner...it doesnt say that in the rules...is there a way to get them unstuck or are they no longer usuable once ya move them there...