Picked up a new to me GSG-5 not long ago.

At the range it's been pretty unreliable. To begin with I was getting a FTE stovepipe every couple of rounds.
A new round would be fed into the chamber but the empty was getting squashed sideways by the bolt.

Using CCI Blazer I was getting a mix of super & sub sonic shots. The barrel is about 8" with the suppressor pinned to be legal (UK law)

I sprayed a generous amount of oil into the action and it started running a lot better, about 70% reliable. It was detail stripped and cleaned prior to going to the range.

I'm thinking I need a higher velocity ammo as it seemed to be the subsonic rounds were when it failed to eject.

Any other thoughts on what it may be?

Yep try higher velocity ammo.

Tried some mini mags but still getting the same result.

Ordered a new extractor, spring and recoil springs.

Have you completely stripped the bolt and cleaned it down?  

Mine gets a lot of grit trapped in the bolt and has to be torn down and cleaned real well to run again.  I have a very early production version which had the un-riveted ejector and had to go back to GSG for the upgrade to run reliably.  Also make sure the chamber is clean and the ramp and edges dont have buildup on them so you can have a smooth pull on the ejection.

Thanks.
I fitted the bolt screw upgrade kit and still getting stoppages.
Will add a new ejector to my order too as mine isn't riveted either.

Edit: I have completely disassembled the gun, clamshell receiver and all. Thing was filthy, I'll report back once it's back together.

New ejector (riveted to carrier half version) and extractor fitted, made a trip to the range and it runs CCI Blazer 100% now

That did the trick for me and many others too.  They get dirty too, its not an easy strip but I always make sure mine is cleaned and well lubed.

Glad you got it back up and running!  You must get some attention at your club with that set up.

Out of curiosity, how much do CCI Mini-Mags go for?  Do you guys see a lot of the low-end "plinking" ammo or more of the European target ammo?

ETA:  I took a tip from Larry's Guns, who is the USA distributor for Hammerli, regarding keeping the chamber of my SIG Trailside clean.  They recommended taking a .25 Caliber bore brush, bending it 90* and using it as a chamber brush.  This is the only way I can run my Trailside for long range sessions.  It still can cloverleaf.

I have a 2008 gen 1 gsg5 and it is completely nonfunctional. Ran great for a couple years with any and all ammo over 1200 fps. After that FTE’s and light strikes were the norm. 2 rounds in a row was progress. I’ve read a lot about the riveted ejector(which ATI is charging to do now) but it doesn’t make any sense because the ejector is held tight in place with the clamshell bolt housing. I pulled it out of the way back of the safe and  begun doing some of the mods(firing pin, hammer strut, mag trigger safety) and a little more tweaking and polishing, but I’m hesitant to pay for the rivet job because it seems the ejector is held tight by the housing.
BTW The only ammo I use in it now is CCI Minimag