Why did David need to call Bas Sheva?
Radak: She had not left the room, but she distanced from David a bit when Nasan talked with him.
Radak citing Chazal: He called her to have Bi'ah with her 1 (to prove that he could have Bi'ah, and the only reason he did not marry Avishag was due to the Isur to take a 19th wife. Avishag had said that he is too old and weak to have Bi'ah.)
Malbim: She had left [the room] when Nasan came.
Rashi (Sanhedrin 22a) explains the Gemara like this, but it learned from "va'Tavo Bas Sheva El ha'Melech ha'Chedrah" (verse 15). Ramah explained that he called her, and amidst her desire for David, she emitted much semen (from their last Bi'ah) in front of Avishag.
