For pistols that have sat for a while
The internals get a little bound up resulting either the bolt not dropping back into the cylinder as it should or resulting in the bolt dropping too soon and the cylinder won't spin on half cock.
Taking it apart, cleaning it up, lubing it and putting it back together is probably a good initial fix for most pistols with that problem.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson