[lugerholsterrepair]

Justin, Often in mismatched sideplates the takedown lever comes up against the sideplate arm that is too fat. This can make it hard to rotate down. You can take out the takedown spring and see if the condition persists. If it does... the sideplate arm under the takedown lever is indeed..too fat. I have filed down my share of these to loosen up the lever.
If it has the affect of being alright when the spring is out of the pistol..it is the spring. The spring is an arcing wire affair that interacts with the rotation of the takedown lever by fitting into a slot. If the arc is too high it will be difficult to move it from this slot. Or it could be a combination of the two.
A good tool to begin moving the takedown lever is the thin end of a clothespin. Non marring wood and generally slips right in.