My thought is....
You are almost going to have to find the pin and take the latch out of the loading lever.
With the pistol all put together you should be able to examine the mating between the loading lever latch and the catch on the barrel. These two parts have to be of the right shape in order for the latch to engage in the catch.
If the latch has enough purchase but just has to be reshaped, your fix is easy. Just file the right angle on the end of the latch and your are done. Similar job on the barrel mounted catch. Has to have a nice ledge which is essentially parallel to the bottom surface on the barrel.
If the latch is not coming out of the lever far enough to engage the barrel mounted catch, you have to take the latch out, file the slot a little deeper, and put it back together. You can see the pin by looking into the end of the lever and just look for it coming down through the slot in the latch.
Be careful when you drive it out. As soon as the pin clears the slot in the latch, the latch will fly out of the end of the lever and then latch and spring will be gone (and in my shop they will be gone FOREVER).