Real Games (tm) let you slide around corners you'd otherwise bump your head on when jumping, like Mario when he jumps with a few pixels of overlap into a solid block above. FISH does this too but in a slightly less player-friendly way, where you lose a frame of jump for each frame spent sliding sideways around a corner. Because the max jump height is only a few pixels more than the amount required to make the jump, any jump height lost to sliding around the corner means you probably won't be able to make the jump. Probably the easiest fix would be to increase the maximum jump height.