Logisim Help - Using Custom D Flip Flop
D flip-flop provided by Logisim which you used for simulation was a positive edge-triggered D Flip-Flop. While what you have designed is a level-sensitive D latch.
You have to cascade two of those D latches in master-slave configuration to obtain a positive edge-triggered D Flip-Flop.

reference: Flip-Flops Wikipedia