Why did Shmuel need to ask? Since Hashem did not tell him yet, surely Yishai had another who will be king!
Malbim: Indeed, he asked in astonishment 1 ; he knew that not all had come.
Perhaps he was astonished why Yishai did not bring all his sons like Shmuel had commanded him (verse 5). (PF)
What is the meaning of "Sha'ar"?
Rashi: He remains. Radak - this is like Nish'ar. This is only the only place where this verb is the simple form (and not a Niphal or Hifil).
Why did David not come initially?
Malbim #1: He was the youngest - "Ra'uni Ne'arim v'Nechba'u."
Malbim #2: He is engaged in his work, grazing the flock.
What is the meaning of "Shilchah v'Kachenu"?
Radak: Take him via a Shali'ach, that he will come to here.
What is the meaning of "Lo Nasov"?