Andrew, if this is a Fulton box lock, was it broken down recently meaning was the for-end taken off and the barrels removed from the receiver? If so it might have been taken down fired. Look with a flashlight and see if when opened as far as it goes if the cocking plate looks like the one in the picture. If it does not, take the for-end off, take barrels from receiver and put receiver in a vise with wood or padding on each side. Take a long thin screwdriver and but it over the hinge pin and under the cocking plate and raise it up till you hear it cock, then put the barrels back on the receiver and then for-end it should open all the way now.
As another check, take for-end off, barrels off receiver and see if you can move the cocking plate with your fingers, if you can it is cocked. It has to look like the picture to work.
The hook part you see on the barrels might be under the cocking plate not letting it open all the way.
If this is not the problem email me and I will try and replicate your problem.