[Gameplay] Twitch auto attack fails with ult on

So i frequently find that when I activate Twitch's ult that people can dodge his auto attacks. I'm not sure if that is what is supposed to happen or not but if it isn't than it is annoying because it has cost me a few games. Here is what happens:

1. I get in position with ambush so enemy has to run past me to escape
2. I activate my Rat-ta-tat-tat and commence the hurt
3. Enemies with high speed begin to run in front of me and my auto attack hits a couple feet behind and completely misses them doing no damage.

If you could fix this or let me know that this is supposed to happen i would appreciate it. I main twitch and currently have lvl 5 mastery with him so i play him a lot especially in ranked games and this bug is annoying. Thanks!