"While Satan was laying his plans, Jesus was carefully opening to His disciples the sufferings through which He must pass—that He would be crucified and that He would rise again the third day. But their understanding seemed dull, and they could not comprehend what He told them. EW 161.2"
"But they understood not this saying, and it was hid from them, that they perceived it not: and they feared to ask him of that saying."
Both passages describe the disciples' inability to understand Jesus' predictions of his suffering and resurrection. The EGW passage amplifies the idea that their understanding was 'dull,' while Luke states they 'understood not.'