Note 3 at Jn. 21:4: There is no explanation given as to why these disciples didn't recognize Jesus in this instance as there was with the two disciples on the road to Emmaus (see note 4 at Lk. 24:16, p. 492).

It was very early in the morning and perhaps the light was not yet bright enough to clearly see Jesus. Also, verse 8 says they were about 100 yards away from shore, which is the length of a football field, and that would be far enough away to cause them not to recognize Him.

However, verse 12 mentions that once the disciples were on shore and face to face with Jesus, none of them would dare ask Him, "Who are you?" because they knew He was the risen Lord. This implies that they still didn't recognize Him by His physical appearance but rather by revelation.

Therefore, it is most probable that, just as with the two disciples on the road to Emmaus, it was the resurrected form of Jesus' glorified body that caused the disciples not to recognize Him.