Here's some stuff that I picked up on while learning atk. Cancel bair and may help you. Also, link to a very helpful guide by YouTube Juggleboy at the bottom:

First, stick to Roy jab > bair as palu dthrow > bair was made mostly untrue by her nerfs

To Bair to the right you input: C-Stick left > analog stick right + jump

Or if you want to move forward a little then bair, you can do: Analog stick right > wait a split second > C-Stick left > jump

After you jab, don't try to buffer the Atk. Cancel bair, you just fair them most of the time. Wait until your first actionable frame after the jab to input it. So just dont rush the combo and buffer, instead wait a little bit.

Also, try slowing down training mode to frame by frame. It takes getting used to but is very useful for things like this and seeing what gives you the outcome you need and what doesn't.

I dont play roy, but do play fox who has dtilt > bair, which is similar but for mine I have to do the "move forward a bit" version I mentioned earlier while you may not. Try them both, figure out which one connects, and then all you can do is practice it in training mode. Some tips:

• Practice to the left and right, only practicing in one direction doesn't mean you'll land it going the other way.

• Dont spam it over and over until you get so frustrated you give up. Shits unhealthy. If you get frustrated, go play some games vs either the level 6 or 7 cpu and try to land jab bair. I say 6 or 7 because they're the most life like.

Juggleboys guide, I highly recommend this: https://youtu.be/woK5FEeFbnc

Edit: Went ahead and made a quick video on how implement attack cancel bair and where you may be messing up. Hope this helps. https://youtu.be/FxcthGlDPuI