In fact, you should focus in using Up Slash / Down Slash combined with a basic 2 hits combos, instead of depending on them alone and isolated to start a juggle and next, a combo ....
For example, I use F+2,1, + Up Slash almost all the time , and follow with a combo, instead of Up Slash alone. I use Up Slash alone just as a normal special move, not as intention of a juggle combo start
For Down Slash, use F+3,1, + Down Slash, and next, the rest of the combo, works better, connects. Using Down Slash only sometimes, not as a proposital juggle combo starter
Doing quickly, all stuff above get chained together and so, you can go next for your favorite combo sequence.
Of course that, when using Up Slash / Down Slash alone, if you see that connected and hit, if you have good reflexes, so, you can take the chance and go for your combo, but, I prefer the 2 basic sequences I told, because are 100% guaranteed, just all you need it's that the very first command to enter sucessful against the opponent.
I use F+4,1 - Down Slash - dash - F+2,1,2,1+2 - Kunai/Port a lot I love that combo and F+2,1,2,1+2 is my fav skarlet's string.