Tian-Tian had the same issue when I first brought him home. He went through some intense recuperation after enduring a major ich/fungal infestation and he never flared once. I didn't know if I had somehow damaged him in the healing process...
Thankfully, he healed nicely and began to make LARGE bubble nests and defending his "territory" from my tablet pen and blue M&Ms after about a month.
He would look terribly adorable and not at all intimidating when he flared.
How long have you had Crumb and has he been making bubble nests, or was he sick recently or transferred to a new tank?