Hello to all.
I'm working on Scalewise motion but when i try to target the chord tones (3rd and 5th for now) at first beat of chord changes, i get lost easily.
Am i missing something? Is there any approach or trick to make it easier?
Another Q: how much is this common in professional walking Bass lines? How much is it important?
As far as the getting lost easily, this is most likely because you are trying to progress too quickly. When I first started learning to walk, I would slow down as much as necessary, so that I could think of all the chord tones and then choose what I wanted to play. I might even play out of time entirely. It was basically a process of experimentation to see what sounded good.
Some foundational material you need: Learn your major and minor keys signatures and all 7th and 6th chords cold. You could also practice arpeggiating the chords in root, 1st, 2nd, and 3rd inversion.
When you are first learning a new tune, consider stringing two bars together at a time and repeating them slowly until you have a variety of patterns you can play. Then move forward one bar and do the same thing. After you have worked on four bars string them all together in a repeat. The trick is to always start as slow as necessary, so you can actually think about the theory you need to play the changes.
Once the song is "played in" you will use primarily your ears and the linkage with your fingers. Basically your mind will eventually automate much of the process. The chords become archetypes that your mind translates into a library ideas that you can apply instantly without a lot of conscious thought about the notes. The concept of ear training is a very important thing to grasp, here as you should hear what you play in your head as you finger the notes.
As you get more skilled you will not need to slow down as much, and you will be able to sight read simple changes at tempo. But some changes will be tricky, and slowing down, performing harmonic analysis, and playing the changes "in" will always hold value. Basically this is necessary when chords are used in a way that you are not familiar with, or when you face chord alterations that you don't intuitively understand. When this happens, the archetype your brain has established for chords does not work properly, so you have to slow down and learn a new way to approach the problem.
Although you do tend to play the root on the down beat more frequently, with some degree of finesse you can play any chord tone on the down beat in the right context, and you don't necessarily have to play the root on beat two. IMHO 3rds and 5ths are so consonant that you can often play them on the down beat and simply walk diatonically to the next chord tone, and it will sound nice. If you play a more dissonant tone like a 7th or a 9th on the down beat, you will probably want to follow it with the root on two. When you play a 7th or 9th on the downbeat you are intentionally making a fairly overt statement. Whether you can get away with this is somewhat dependent on the style of music your are playing, and the players you surround yourself with.
Good Luck!