Was this guy a fish who didn't see the flush? Did I not bet enough on the turn to convince him I had it?

Or am I just a bad poker player altogether who should not bluff at a pot ever again?

Just sat down at the table maybe 10 hands earlier.

Party Poker
No Limit Holdem Ring game
Blinds: $0.50/$1
6 players
Converter

Stack sizes:
UTG: $45
UTG+1: $59
CO: $6.75
Button: $0.45
SB: $3.75
georgtbone: $25

Pre-flop: (6 players) georgtbone is BB with 4 K
UTG calls, UTG+1 folds, CO raises to $5, Button folds, SB calls, georgtbone calls, UTG folds.

Flop: 6 5 3 ($16, 3 players)
SB checks, georgtbone checks, CO checks.

Turn: J ($16, 3 players)
SB checks, georgtbone bets $15, CO calls, SB folds.

River: 3 ($46, 2 players)
georgtbone bets $30, CO calls.

Results:
Final pot: $106
CO balance 9.75 bet collected 3 net Jc Qd
georgtbone balance 25 lost 4h Kh