Since I consider myself an expert on the DT3, I'll answer this...

Notice the space between the back of the pistol and the arrow launcher in back. Make sure the two guns are tightly fit together at the back. Notice up top, the latch is not meeting trigger catch.

The reason this is happening is that your pistol barrel is not the same diameter as the front orange piece with the spring. This piece forces the pistol back into position. The orange piece is over your barrel, therefore it can't push it back into position. I suggest going back and doing my DT3 mod. That should solve your problem.
Also, there is no need to have a long barrel on the DT3 pistol. This just makes it impossible to dissengage the pistol from the launcher.
Hope this helps.