door knob won't unlock/unlatch

Front door knob won't open, latching mechanism not unlatching, etc. Lock it, unlock it, use the key, etc., nothing turns the latching mechanism. We managed to get the latching mechanism out of the door and door frame and can use the dead bolt for now.

This in the second time it has happened in about 3 years. never happened in over 40 years of home ownership. What is going on?

Did I buy a cheap replacement? Three years ago I just went to the local hardware store and got something similar off the shelf? Do I need to buy a better quality door knob? I don't need heavy duty stop the marauding hordes type of door lock. (My pit bulls would lick you to death, all l bark, no bite.)
Any standard front door knob recommendations?

 

Pictures of the latchbolt you removed from the door and the strike still on the jamb. They may help us identify the problem.

 

Thanks for replying, my neighbor came over this morning and sorted the problem.

 

So what was the problem? Inquiring minds want to know...