I have a bed in front of my house (part sun, northern exposure) where I've
tried some annuals that are supposed to reseed (marigolds, sweet allyssum,
coreopsis, and something else I can't remember now), but nothing ever comes
back. I'm wondering if there's something I should be doing to encourage
them, short of collecting seeds & sowing them the next spring.


Do the annuals' seeds contact bare soil around your annuals? I wonder too,
how many hours of sunlight you get with your northern exposure? The annuals
that you listed generally do best in a sunny location.

Keep in mind that many self-seeding annuals can appear where you don't
want them. I have had Cosmos (Cosmos sp.) and Lobularia maritima (sweet
alyssum, sweet alison) self-seed between the interlocking stone pavers.
Myosotis sylvatica (forget-me-not) and Calendula officinalis (pot marigold)
are appearing in unexpected places too. Fortunately Iberis umbellata
(candytuft) have been better behaved when self-seeding. I have all the above
in a mixed sun/shade, except sweet alyssum which has mostly sun exposure.

If you go this self-seeding route, choose something like cosmos whose early
seedlings are easily identified. Without a mulch to keep the weed seedlings
down, there will be extra work weeding around your volunteers. Free plants
can cost a lot of time.

Regards.