Take the lower tang completely out of the rifle so you can see everything that moves. Then cock the hammer and see what the hammer is running into. Cocking with the lever has HUGE mechanical advantage that is missing when you thumb cock. Then closely watch what the trigger assembly does.
If the hammer has full travel to the rear, then the problem is in the trigger assembly. Something is preventing the upper (sear) portion of the trigger assembly from engaging the hammer.
With a bright enough light, you may not have to remove the lower tang. Good Hunting
PS: If the same mistake exists every time you take it apart and put it together, it will get real frustrating (you know that already). So look carefully at the assembly to see what it is doing, and more important, what it is not doing. You don't have an after-market trigger in this rifle do you??