Serama aren't known for being consistent layers. Some hens will lay well, others won't. Some hens may be so small they'll never lay. My girls will lay only about 10 or so eggs before they go broody again between clutches.
As far as being cold tolerant, I've read that because they were "created" in a climate that is warm year round, they don't do well with the cold, but there have been many breeders that have found they tolerate it a lot better and are a lot tougher than they were lead to think. Here in Texas, mine don't get any special treatment, since it doesn't get very cold here in the winter. I just make sure they have shelter and aren't exposed to severe weather.