Mark Chapter 06

Mk.6:51And they were greatly amazed in themselves beyond measure, and marveled.

In german translation it says, that they were more shocked than amazed ("und sie entsetzten sich über die Maßen"). And the reason for that is explained/mentioned in verse 52(Mk.6:52 For they had not understood about the loaves, because their heart was hardened.).
Although they have seen this great and amazing miracle from Jesus, they did not understand. That's why Jesus also says:

John20:29 "Blessed are those who have not seen and yet have believed.”