jugleong
Sorry to hear about the issues you've been having.
Usually Canaries will have more than one egg in a clutch, but one egg is not unheard of. Is your girl sitting on the egg?
I would recommend a diet of good quality Canary/Finch seed mix . Along with that, I would offer grated hard boiled egg on a daily basis (half an egg should be fine for one or two birds, and you can refrigerate the other half for the next day). Greens, like spinach, kale, broccoli, cucumbers, parsley, beet greens, romaine lettice and other similar vegetables, should be offered in small amounts daily. Also some apple, orange slices, banana slices, are very good and enjoyed by Canaries.
It's very very important to offer cuttle bone, crushed oyster shell, and crushed egg shells from boiled eggs to your birds. These offer calcium, which is extremely important for your girl (especially if she is laying some soft eggs).
If you can find a mineral block for your birds, I would offer that as well, along with bird grit.
A dish of lukewarm water every day for a bath, would be appreciated.
Do you by any chance know how old your birds are?