If they have been outside some time it may have been too cold for them. If in pots under cover they can rot where the stem meets the soil....damping down. I have only just put my tender plants completely unprotected at night, last night for the first time,after an extended period of hardening off......that is gradual introduction to colder temperatures and wind ,so stems can get tougher. Once they are hardened they can tolerate quite a lot of hardship.....Welcome to GoY Corgi

Tobacco budworm may be a problem, too. The young caterpillars burrow into the flower buds, which then turn brown and shrivel. Sometimes you can see the hole where they went in or out, sometimes not. Older caterpillars will eat holes in the young leaves. B.t. is a good preventative, and spinosad or imidacloprid get them in the bud.