What are the most likely causes of stovepipe jams?

CS40 with not a lot of use. Seems like it would jam one maybe out of three mags or so.

Thanks.

Usually failure to have/use the proper grip (limp wrist), or problems with the extractor. A dirty chamber, breechface (etc) can also cause FTE.
Start by doing a cleaning, then check the tension on the extractor and for any damage to the "claw".
Ammo also plays an important part in the malfunction.....improperly re-sized handloads, damaged cases, etc.

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Ret. LE, FA Instr, S&W Armorer

I shouldn't have assumed you were talking about only ejection problems. FTF issues can also be called "stovepipes", where the live round ends up trapped in the port, or between the barrel and the breech as the gun tries to close.
In this case, I would suspect damage to the magazines, or weak magazine springs. Sorry, should have included this in the other post.

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Ret. LE, FA Instr, S&W Armorer

To clarify, the issue was empty cases, factory ammo, clean gun, firm grip...

Is too stiff or too weak a recoil spring an issue?

Yes....they do have a direct effect on the slide velocity. (opening and closing) I would suspect a weak spring first.

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Ret. LE, FA Instr, S&W Armorer

Yes, either can be. Stovepipes are a slide timing issues, weak ammo with stiff spring, etc.