John 6:60 is the only record we have of any of Christ’s followers forsaking him for purely doctrinal reasons. If it had all been a misunderstanding, if they erred in taking a metaphor in a literal sense, why didn’t he call them back and straighten things out? Both the Jews, who were suspicious of him, and his disciples, who had accepted everything up to this point, would have remained with him had he said he was speaking only symbolically.
But he did not correct these protesters. Twelve times he said he was the bread that came down from heaven; four times he said they would have "to eat my flesh and drink my blood." John 6 was an extended promise of what would be instituted at the Last Supper—and it was a promise that could not be more explicit.
Then how did the thief on the cross recieve salvation without ever eating this physical flesh? He did consume Jesus spiritually and was told as a result he would dine with Jesus this day in paradise.
There were many times Christ was speaking in a spiritual or symbolic sense where He didn't 'straighten out' those who misunderstood what He was saying. And sometimes, when He did 'straighten them out', He only did that for a select group, i.e. the 12.
By the way, do you think the 12 understood all that Jesus taught at the time He was teaching it, and where they didn't He 'straightened them out'? No, of course not, or they wouldn't have all been hiding in the upper room after He had been crucified. They wouldn't have been so surprised when the women came and told them that Jesus had arisen.
Jesus' death on the cross and His subsequent resurrection was spoken of by Him more frequently than communion/the eucharist, and yet He didn't 'straighten out' His followers about that.