This just started today. I have to turn the knob in order for the latch to retract. Before, I could just push to door shut and the latch would retract when it hit the plate on the door frame. But now it just hits hard against it. Other than that, the door knob works perfect.
You may have a deadlocking latch bolt, which in addition to a normal door latch has a plunger. When this plunger is retracted (which happens when the door is closed), the spring loaded mechanism on the latch is disabled. This prevents someone from using the credit card trick to open the door from the outside, so the only way to open the door is by unlocking it and turning the knob. In other words, it's for security.
Sample image from doorware.com, no affiliation
My guess, assuming this is an external locking door, is that this plunger is either jammed or some of the internal mechanisms that it activates have jammed. You can unscrew the door knob and remove both sides, then unscrew the latch and pull it out of the door. From there, try some lubricant (silicone spray and graphite powder are my preferred lubricants for locks, WD-40 will quickly dry and attract dirt). And if it still doesn't work, it's probably time to go to the store to pickup a new doorknob.
The exact same thing happened to us last week. We called Schlage, the manufacturer, and they said that part of the internal mechanism was broken. Because they warranty the mechanism for life, they sent us a new mechanism with instructions on how to install it. It arrived just a few days later, and installing it was a simple task. Now we can just push the door shut again without having to turn the knob.
The answer provided by @BMitch is good, but here's another suggestion:
Check to see that the plate on the door frame is actually making contact with the angled part of the latch.
It's possible that due to thermal expansion or contraction of the door or frame, the plate is now striking the latch on the non-angled part causing it not to retract.
The latch bold was sticking on my front door. It's a schlage dead latch. I called the company after fiddling with it. Something had clearly gone wrong with the latch itself. After a little time on hold I got through to customer service. She was able to determine what model I had because there is no model number anywhere on the thing. Long story short my product and possibly yours has a life time warranty on parts. She put the order in and true to their word 5 business days later the latch showed up and I just finished installing it. No problems, it's not hard to do. FYI Customer service number is 1-888-805-9837.
and there was no charge.
The part you are refering to is a hardened pin designed to prevent someone from cutting the latch with a hacksaw to gain entry. It is a part of the backset mechanisim. The backset will need to be replaced. Most are lifetime guaranteed for mechanical defect, and commonly replaced by the manufacturer, or purchased at most building supply stores. I work in such a place on Vancouver Island in Canada. This is a very common problem, with a very easy remedy.
The tongue of the door latch leading from my garage to the kitchen began malfunctioning just as described above. It would retract when either knob was turned, but not when attempting to close the door without turning a knob. After reading everything written above, I concluded that I was looking at replacing some or all of the internal mechanism. With that realization in mind, I decided to just hit the thing a few times with a hammer. Unbelieveably, that seems to have worked, and now the tongue isn't sticking any more.