Your sear spring has a small projection sticking out of the side of the upper area of the leaf.
It should be in the slot in the lockplate to keep it from turning.
My method of assembling one of these is to place the spring on the lockplate with the lower end resting on the sear.
Then, I insert the screw and run it in until it is just touching the springs eye.
With my thumb, I press on the rear of the spring where the U is, forcing it downward and towards the lockplate. After it moves down just a bit, the projection will snap into the lockplate groove. I then tighten the screw to hold everything in place.
The screw which passes thru the sear (or on the bridle at the sear) should be just "snug". It should not be tight.
If it is overtightened, the sear will not move freely so it may not move into engagement with the full cock notch or the half cock notch.
It will also be difficult for the trigger to move giving the appearance of a very heavy trigger pull to get it to fire.
The other screw(s) should be tightened.
Do not unscrew the nipple. It needs to be snug against the drum or bolster in order for it to fully seal off the high pressure powder gasses.
The caps may be defective or, more likely, the nipple cone is slightly too large for them.
If the cone is a little large it will prevent the cap from fully seating so that the priming compound is resting on the top of the nipple.
If this happens, the hammer ends up pounding the cap down until it does finally seat on the end of the nipple. The next attempt to fire the gun usually results in the cap firing.
It is difficult to determine if this is happening but generally speaking, the cap should not be a tight fit on the nipple. If it is fully seated and the priming compound is resting on the end of the nipple, you should be able to rotate the cap with just some strong finger pressure. (It's difficult to explain but the cap should not easily fall off of the nipple but it also should not be tight.)
I don't know what kind of solvent your using but plain water and some dish soap should be all you need to fully clean the bore.
Have fun.