I wouldn't know, but I have a feeling that she didn't have to do those roles and, basically, did a lot of walk-ons fo111cr her friends, like Ford. She was relatively comfortable and didn't need the money or adulation. She is always recognizable, though. Her short speaking roles in film like "Two Rode Together" and "The Robe" show that she certainly hadn't lost her talent.

But there are movies where uncredited performances are remarqued by the public and are an honour made by the director(Even recently, Jean-Pierre Leaud in Alphaville for example. Everybody who watch it recognize him).
Isn't it the reason why she made so muxh uncredited actings with John FOrd, for example? I couldn't believe Ford thinking: "OK, I'm going to engage this woman, but I don't want her name to appear" or "her names not important" so many times.
I'm sure he wanted her to play in those movies.

No disrespect to Ms. Marsh, but it looks like a lot of those parts make her look like a glorified extra, in some cases. She had a silent film career that was rather good, but it didn't carry over into the talkies. That success didn't work for a lot of people, so there isn't much you can do about it.
About her later career
When you see 5 or 6 pictures a year, then she WAS an extra, it seems. Most of those picture were produced by her stepfather and she was quite easily accessible for a producer. But, when you see only one picture for a year, she was doing it as a favor for the director.
I could be wrong with my assessment of her career in the 30s-50s, but I would need someone to back me up on that.
"It's so hard having a good time. Even smiling makes my face ache." - Dr. Frank-N-Furter

that's just a way of earning a living, in those day, many supporting actors are uncredited, not like today, even soldier A or B is usually credited.also no independent movies for she or other "out " actresses to star in

Is she the one who spits at the ashes in "Donovan's Reef?" She's just darling!
I admire a person that's willing to do whatever is necessary.

This message has been deleted.

she probably couldnt enter the talkies, or was too underratedit is kind of sad a star like her ended up doing so many great films but appearing uncredited on them!!
Mientes tan bien, q me sabe a verdad todo lo q me das..