This is because you DO have to walk forward. After the fierce and jab slash, you need to walk forward just a bit in order to get yourself close enough so that you can connect the slashes afterwards. It might work randomly otherwise, but if you walk a certain short distance, it will work every time. This is character dependent, as characters such as hugo, alex, and standing makoto are really fat and require no movement.
The reason it looks like you’re in range but you’re not hitting is probably because the sprite is stretched a little thin where your fierce slash is supposd to hit. When they start to recover it gets back in the right place, but they often block. It could be also because the slashes have several hit frames and the range changes between them. Hitting later during them may be too late and allow them to recover.
It’s much easier if the opponent is crouching, which gives you a lot more time to walk forward. It’s very short, you basically have to stop as soon as you see him walk forward (unless they’re crouching) and hit the chain.