I had the same problems you are describing. My UMP in .45 worked 100% but when I changed over to 9mm, using US made parts it was not so great. I tried the swapping out ejectors with an HK factory one and it worked a little better but nowhere near as well as the .45. The only thing that ended up getting everything working for my UMP in 9mm was I put an original factory HK 9mm UMP bolt (complete factory HK not a copy, modified by TommyB to fit Omega UMP receiver) when I put that in my 9mm UMP it ran just as well as it did in .45 100% perfect. I don't know what makes the German bolt work and the USA made bolts jam but the proof is in the firing. Now finding a HK made UMP 9mm bolt is going to be hard and pricy when you do find one but it was more than worth it to me personally to have my UMP in 9mm it's a very nice gun in that caliber much smoother operation than the .45.