Contradiction with Matthew 7:7
Matthew 7:7 states "Ask, and it shall be given you," which seemingly contradicts John 12:21 where it's not specified whether the Greeks' request was granted.
Matthew 7:7: Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you:
Contradiction with Matthew 10:5
Matthew 10:5 records Jesus instructing His disciples not to go to the Gentiles, whereas John 12:21 involves Greeks (Gentiles) seeking Jesus.
Matthew 10:5: These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into [any] city of the Samaritans enter ye not: