Did the Companions understand the interpretation of all the verses of the Qur’an, or was the interpretation of some of them unknown to them?

The Companions understood the Quran by their very nature, because it was revealed in a clear Arabic tongue, and they would ask the Messenger about anything that was unclear to them. Some of them might not move beyond ten verses until they understood their meaning. Ibn Abbas interpreted the entire Quran; Mujahid ibn Jabr presented the Mus'haf to him three times, asking him about every verse. No one possesses infallibility after the Prophet, peace be upon him, so some meanings might have been obscure to some Companions, or they might have erred in them.