though you cant just stick eggs under any chicken they have to be broody.
I mean that seems obvious but just makeing shure , thats the kind of thing I would end up doing.
Only some go broody and not all the time. Certain breeds tend to be more broody than others also.
When an individual hen goes broody she will sit on the nest all the time and not want to get up. Mine who were broody would be sleeping in the nest instead of on the roosts with the others.
If you keep taking the eggs away from broody hens most will give up.
I had one persistent girl and finally separated her and put her on some eggs and she was great, no problems. Sat on the eggs fine for the whole 21 days. Is now a devoted mother hen. She is a bantam Dutch.
Seems to be individual quirks as to if/when go broody but leaving the eggs in the nest (or good fake eggs in the nest) encourages broodiness.
What kind of hen(s) do you have?