The retainer which is supposed ro hold the pin in place is not in the groove on the pin.
Without knowing what model rifle you have it's impossible to be more specific than that.

Here's info on the hammer and trigger retaining wire as used on the AK.

There are several types of wires, and you can replace the wire system ith one of the pin retainer plates.
The wire is a little hard to work with, but it's an "elegant" solution to the problem of retaining the hammer and trigger pins.
Once properly installed it's fool proof.

Note the link on the lower left of the page on "Fire Control Removal". That explains the wire retainer.

http://www.novarata.net/Linx310/whyromanian.shtml