Key slipping in rack mortice door bolts

The key slips on some of the rack mortice bolts on our doors. In about half the bolts the key slips and in about half it doesn't.
On some bolts it slips at the end of its travel in one direction but in the other direction it is firm at the end of its travel.
Does the slipping indicate wear of the mechanism and that it needs replacing? Or is it just a large tolerance from poor assembly and not a problem?
This is what I mean:
http://picpaste.com/Y8002_image.jpg
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On 20/04/2017 12:37, pamela wrote:

We have those, and some of ours slip too. There are two windows that I can't open. I recommend changing them soon, as it's easy to do whilst you can still open the door!
You could try a new key, in case that's worn, but it's likely to be the rack inside.
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On 13:03 20 Apr 2017, GB wrote:

In that case I'll get some replacements from Amazon. Are these rack bolts a standard size or will I need to specify the diameter/length?
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On 20/04/2017 19:09, pamela wrote:

I think they come in a couple of different lengths, one for doors and one for windows.
But, I'd remove them and check the length. Also, the position of the key hole!
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