Word for not being able to focus on one task

The word closest to the examples you already gave is scatterbrained:

[Merriam-Webster]

: having or showing a forgetful, disorganized, or unfocused mind : having the characteristics of a scatterbrain

"Distracted" is the word you are looking for, where you are not concentrated. That's when while doing one task your mind reacts to what your eyes are watching and what you are thinking, you get drawn away from your task, and you get distracted by other things.

scattered

I can't find a good dictionary definition of this, but this book title shows pretty clearly how it can be used for your concept:

Smart but Scattered: The Revolutionary "Executive Skills" Approach to Helping Kids Reach Their Potential

all over the place

This phrase works great for the context you're interested in. Example:

John's comments included some interesting points, but he was all over the place and in the end he didn't help our case. I think we should leave him off the speaker list for next time.

Finally, the most obvious word:

unfocused

(needs no explanation; usage: the same as "all over the place")

Edit (4/23): There is another one gaining traction in the US at least:

ADHD (as an adjective)

This is an acronym for a neurodevelopmental diagnosis, Attention Deficit/Hyperactivity Disorder, which is a subtype of ADD, Attention Deficit Disorder. Example showing how it's starting to be used in the US:

Prof. X is a good teacher but if you want to get questions answered in office hours, make sure you bring a written list of your questions with you. Keep coming back to your list if he goes off on ADHD tangents.

Be careful with this one. It hasn't lost the stigma everywhere yet.

The person is:

• absentminded;
• inattentive;
• miles away (informal);
• in a world of his/her own (informal); and,
• (maybe) daydreaming.

I like the above suggestions better, even though, per any dictionary, these all are synonyms of 'distracted' which already exists as an answer in this thread.

to wander TFD

(of the mind, thoughts, etc) to lose concentration or direction

As in:

He frequently wandered off the point during our conversation.