Honestly it was probably a mistake on Kishi's part but he will probably come up with a justification like Kurama took it while Naruto and Gaara were fighting, or Shukaku left some chakra with the other tailed beasts to give to Naruto.
When the rest of the TailBeast retired to the Juubi they told Shikoku,you must remember Gaara is alive the First Tail Had no Host so it stayed in the Juubi the Tail Beast can speak to each other.