Assuming you have the correct key etc etc....
This can be due to a worn key, do you have a second one to try?
Can also be caused by a sticking 'lever' in the barrel, a good douse in WD40 and repeated insertion of the key might free it.
Can also be due to worn levers in the barrel, it can be fixed but first you need to remove the barrel which means that you have first to be able to turn the key....catch 42.
If you can get the key to turn then remove the barrel and dismantle it, look for one (or more) of the brass levers sticking up very slightly when the key is inserted, a fine swiss file or sharpening stone can be used to trim the lever flush with the inner barrel.
Or just get a new set of locks and keys.
I have a spare barrel which has this exact problem, I will try to get some pics of dismantling it and the fix, it seems to be a fairly common problem so perhaps a maint. guide is in order.