I suggest NOT taking apart the trigger assembly.
You can take it out by removing the pins and pull it out no problem, but I don't recommend taking all the guts out of it. Some of the parts are a pain to get back together. You'll be able to troubleshoot the trigger group in its' assembled state once you pull it out of the receiver.
When you do drop the trigger assembly and trigger guard out of the receiver, look to see how part number 15 and 63 look.
The little tab on the rear of the trigger are known to break off when some people tighten the stock bolt too tightly, or put the wrong length bolt in there when they install an aftermarket stock. If the tab is broken off of the rear of the trigger, the broken piece may be rattling around inside of there causing problems.
And, the little tab wont' engage part number 15 and lift it up and at that point, the safety will no longer function properly.