I've created a villager breeder this morning using this video in minecraft 1.16 java edition.
Everything seems to work correctly (the beds assignments are right, the farmer throws his excess to the others villagers).
But my villagers don't breed. I see heart particles at least 1 time a day but they always end up with storm particles. (when I say always I mean about 10 times in a row)
I don't know if I'm just not lucky or if there is an issue with my villagers. (and I accidentally hit my villagers so I don't know if this could influence their breeding rate but if it does feel free to tell me).
Best Answer
Stormy particles mean some preconditions are missing, and in particular, most probably, the precondition is lack of free beds, unassigned to any villager (free for the baby to take, except the baby falling down loses the "connection" freeing it for another.)
What you should do is, during the day (when the villagers stand in their cell) break all the beds, and replace them - first making sure no-one can pathfind to them (break any scaffolding / temporary access that let you access this location, and after replacing the beds break the access to them from the "outside" before restoring your safe path out.