Using this guide, I'm going to walk you threw a FCG pull to check for a blown primer/debree below the trigger.
www.ar15.com/content/guides/assembly/lower/#trigger



With just the lower in hand, use a punch and push the hammer pin out of the receiver while holding the un-cocked hammer in line of the pin/holes. Once the pin is out, the hammer will lift out of the receiver.

Now with the trigger at rest, put hard thumb pressure on the ejection side of the selector, and turn the selector in between safe and fire. You will have to press hard (don’t use a punch/hammer), but the selector detent will ride out of the selector groove and the selector will pop out. The selector detent is stepped, so it will stay in the receiver.
Note: if you can't get the selector out, the trigger will still come out, but you will need to pull the trigger forward to clear the selector, so don't bother pulling the pistol grip to relieve the detent pressure to pull the selector if you don’t want to.

Next, you will push out the trigger pin out of the receiver, and the trigger and disconnector will come out of the rifle.

With the FCG parts out of the receiver, you can now check for blown primer pieces/debree in the receiver cavity, and dry fit the pins to the parts to make sure that you don’t have any binding caused by milling burs.

When you put the lubed FCG back in the receiver, the disconnector spring large coil side goes down into the trigger slot, and the hammer spring legs rest on top of the trigger pin, with one of the legs locking into the annular groove of the hammer pin at the trigger side C cut sides (trigger/hammer pin has only one off set annular grove, so one spring leg will not be in a pin groove, but will still be on top of the pin).




Now back to the hammer pin,

The hammer pin is retained in the hammer/receiver by the hammer inner J spring.  On some hammers, the J spring is too wide or the tension weak, and on some pins, the middle annular grove is too narrow.  If you find either one, then you have the option of calling RRA to get replacements  

If you want the quick fix in the mean time while waiting for replacement parts, with the FCG parts outside of the receiver, lube and install the pin into the hammer (hammer J spring in the annular groove), then use a drill to lightly chuck up on the end of the pin and spin the pin in the hammer while holding the hammer. This will mate the J spring to the pin groove, and mate the pin to the hammer pin channel as well. If you find that the trigger pin is binding in the trigger, you can use the same method to mate the two as well. Also, its a good time to dry fit the trigger, disconnector and pin to make sure that the disco is moving freely in the trigger slot (still will be under tension due to the disco spring).

Big notice: you only want to spin the pins for a second or two. The lube is being used to control heat/ friction and if you spin too long, you will create too much heat and the J spring will lose its tension.

Quoted: Double check your hammer. It sounds like it lost its J-spring (which would result in the pin walking out...and you finding a similar bit of wire under the FCG)

Good call,
It may be that just the bottom end caught the front of the receiver if it was walking out and may have snapped off (the 1/4" section).

The J spring is inserted into the hammer from the bottom, with the bent end in the bottom groove.  To retain the J spring in the hammer, the upper spring leg is peen'd in the hammers middle hole (just above the pin channel).   If the hammer pin did take some force to glide past the J spring when inserted into the hammer, then the upper section of the J spring may still be in the hammer, but since the lower anti retention bent section is missing, the straight leg portion of the spring in the hammer may be turning and allowing the hammer pin to slip/walk out.

Get a new complete hammer from them instead of just a New J spring.  
Getting the J spring peen'd (double punching) without snapping the spring does take a little tallent.