In Skin Deep, he let down his guard and was not clinging to the curse until he realized Belle could break it.
Belle explained why that was the last time it broke the curse to Hook in 502:
Hook: True Love’s Kiss. I need to know why it didn’t work for you and the bloody crocodile.
Belle: Why, it did work, the first time. It awoke the man behind the beast. But Rumple got scared of a life without power, and he chose power over love. He pulled away from me, and in that moment, the darkness forever regained its grip on him. You know, a curse isn’t a curse anymore when the afflicted wants it.
That’s the canonical according to GoogleBelle reason that TLK only has worked once between Belle and Rumple.
They’ve apparently added this layer that Rumple cannot break HER curse without breaking his own…which in my headcanon about what True Love actually means makes sense. Most of the TLK couples we’ve seen have been in a mutually sacrificial part of their relationship–willing to sacrifice for the other–and I think that’s important…so Rumple may HAVE to be willing to sacrifice his power for it to work. But I’m not sure that’s a necessary part of the kiss itself; I think it’s a necessary part for THEM because his clinging to power has been the chief thing between them since they were married.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing