SOMETIMES you have to grind a little of the front of the disconnector where it rests on the trigger. You see? The spring pushes it up, and it pivots on the trigger pin and rests on the trigger. There is a little part that keeps it from going too far forward. If you remove a little metal there, the disconnector can more engage the hammer a little more.
Your receiver could have the hammer and/or trigger holes slightly off causing this. I have seen this before, resulting in the problem you are having.
OR, the front of the trigger, or the hammer sear surface could be excessively worn.
But if you replaced the hammer AND trigger AND disconnector several times, this is not a factor.
With the upper half removed, cock the hammer by hand. Pull the trigger while catching the hammer in your fingers. Now, while holding the trigger, push the hammer back. It should catch on the disconnector.
Now, let go of the hammer. Slowly release the trigger, and the disconnector will let go, and the hammer will be caught by the trigger. If not, there is a problem! The disconnector needs to stay engaged with the hammer a little longer on the release.
So, work these pieces by hand, slowly, watching how it works, or is supposed to work. Compare how they work to a properly working AR15.