On an internal door, the deadlatch does not fully retract into the door
when the handle is turned. It protrudes about 5mm and fouls the
strikeplate when trying to open the door.

There are several of these in my house which are starting to get right
on my nerves.

What is the problem?

I guess I could chisel out the strikeplate, but I know I'm crap at doing
that. Shouldn't the deadlatch retract fully? I can't see that wear and
tear would affect it that much. Would a new handle fix it?

Thanks for any advice.