If I remember right, the top lever/spindle is kept in place at the bottom end with a nut. The end of the spindle being threaded.
You can crank the nut down way too tight,,that just pulls the spindle/top lever down even tighter than necessary for needed functioning and makes for hard lever movement.
The spindle has an off set pin just underneath the toplever that engages the lockingbolt to one side of center (left).
Opening the lever pulls the bolt back and allows opening the bbls.
A V spring common to toplever set ups keeps things in tension.
An extra strong replacement top lever spring may be the cause as well. Or one that is slightly too long on the one leg that engages the spindle and binds it when you attempt to open the lever.
That Nut on the end of the spindle shaft is likely a multi grooved affair needing a spanner driver to sit over the shaft to get at it to loosen/tighten it.
You should be able to see all of this with the stock off and trigger plate removed
