The hen needs lots of calcium to make the eggs strong and have healthy babies. The process takes a tremendous toll on her.
All of your birds need oyster shell for strong eggs. It is very important that you store it in a container that will keep it perfectly dry. If it gets wet, it can grow a fungus which is fatal to birds.
I offer oyster shell daily in a separate dish from the rest of the food and put it out is small amounts and change it when it looks soiled.
I don't know if that is why your fantails aren't laying but if they are opposite sex, that could be the reason or part of it.
Another reason the hen may not lay, if you do indeed have a hen, is because of a coccidia infection. Coccidia is a single cell parasite and most birds have the parasite in their body. In times of stress, it can take over. Most often they will be fluffed and quiet but it can stop them from laying.
Also, you haven't had these birds for very long and maybe they just need time to settle in.
Do you know if they where taken form other mates?
What ever the cause or reason, they do need oyster shell. You can buy it at the feed store in #50 bags. It isn't expensive and will last you a good long time. All the birds will thank you for bringing some home.