It's just my take on the problem, but the Curtis forend release has very little purchase on the sides. It could possibly have so much pressure on it that it is not allowing the clamp to release. To relieve this pressure, try squeezing the forend tight to the barrels while pushing the button. It could even be rusted in place and wiggling the forend in and out and side to side may break the clamp loose. A side to side motion may be more likely to dislodge it.