Try looking at the outside and inside of the receiver....the full length of the recuperator-spring and plunger "hole" to see if a "stop-pin" has been removed or sheared off. Replace as needed.
I believe that some of these could possible have been drilled all the way through the MG34 receiver.
......but photos would still be nice.
Last.......if there was never a "stop-pin" to limit the length and allow for spring-pressure............you can always make a hole and add a "stop-Pin"....."or" make a shop steel pin to put behind the spring to take up the extra space.
US Army, Retired