The problem is first round loading by via the charging handle, and either the mag is not fully seated in the mag well against the catch, or the rounds loaded in the mag have a problem staying to the top of the mag against the bottom of the feed lips.


Starting with the mag release button, make sure that the threaded end of the mag catch assembly is flush with the face of the mag release button.
If needed, shove the mag release button all the way into the receiver, reach over to the other side of the receiver and thread the catch one more full turn.
If you find that the tip of the mag catch threaded section is either a half wind in, or a half wind out with the mag catch assembly back in the slot, then spin the entire mag catch assembly off, turn the mag release button 180* around in the receiver channel for it, and then reinstall the mag release catch again.



Next lets move on to the mags, and they first need to be stripped down to be CLP cleaned, including the inside of the mag body as well.
Note, if you can't get something like BreakfreeCLP in your location, check for Ballistol CLP instead (read CLP is CLP, and it does not matter who is branding it under their own branding name).
http://ballistol.com/

On mags, if you take a good look at the mag, should be able to figure out how to remove the bottom plate to pull the mag apart.
Also, when you are putting the spring back in the follower, the top loop always goes forward towards the front of the follower (bullet side).


Now where you load a mag with ammo, you need to watch for two things.
The first is the mag is fully loaded when the top last round can be forced down about 2/3 of the way of almost allowing another round to be added.  I bring this up, since some of the anti tilt mag followers will take up the space of 30'th round, and the mag when loaded correctly will only allow 29 rounds to be loaded into it instead.

The second thing on the mags, once you have it loaded correctly, rasp the back rim side of the mag body against either your hand, or something solid.  What this does is moved all the rounds in the mag rearward to stack all the rims sides of the rounds in the mag tight against the back side  of the inside of the edge to start with.  As the rifle is self cycling, the b/c coming back over the top round in the mag is pulled backwards again, and has it sitting correctly against the bottom of the mag feed ramps for a clean strip/angle of attack on when the bolt moves forward to strip that round out as well.

As for mags, in your area, you should be able to load up on HK Steel mags that will fit the rifle, and suggest that you do so.   You may be able to find some cheaper clone mags, but even like this side of the pound, its a crap shoot if they are going to run correctly or not instead. Hence on the clones, they may are may not sit high enough in the mag well once locked into place, the follower springs on them may be weak as hell to start with, and even the mag feed lips not formed bent correctly as well.

As for your Oberland rig, here in the Colonies, we would kill to have a rifle that is built to its standards.  Instead, we have Colt rifles as the gold standard instead; which are still produced to sell to the goverment for the lowest  lot bid price in general instead.  Simply, if the rifle is cleaned correctly, and it upper bearing areas lubed with CLP, then the problem you have it either the mag catch not installed correctly against the mag release button, or Magazine problems alone.

To add, if you charge the rifle by hand via the charging handle, do not ride the charging handle back forward.  Just pull it all the way back, and let it go!!!