Usually, Pokemon who have a evolution that require happiness to evolve between their first and second evolution stage, some type of Incense is required.
For this case, you got Chansey because your Blissey was most likely holding nothing or something other than the Luck Incense. That is, next time you're breeding, have your Chansey or Blissey hold a Luck Incense and it will produce a Happiny egg. If you don't have one yet, you can find the Luck Incense in KoniKoni City from one of the sellers.
Hope I helped! :)
