Often they'll lay yolkless eggs when they first start to lay and sometimes when they come back into lay after moulting, but if it's more than just a few eggs, then I'm not sure. If there's no yolk, that could mean a couple of things. Firstly, she may not be ovulating, but if that's the case, I would assume that the hormones wouldn't be there to stimulate the rest of the eggs making process. The other thing I can think of is that instead of the yolk falling into the fallopian tube, it's falling back into the abdomen. If that's what's happening, it can make her quite sick. They end up with egg yolk peritonitis and it gets very ugly. Hopefully it's not that! If no-one else has any suggestions, keep an eye on her over the next few weeks to make sure her abdomen doesn't start swelling. If she does get egg peritonitis, she will start waddling like a duck as her belly swells. I don't know how long it takes to get to that stage as I've never had it happen to mine, but someone else might be able to tell you that. There's 3 treatments for EP. You can take the bird to the vet and get antibiotics and have the fluid build up drained, but that is only a temporary treatment. They need to be drained every couple of days. If the fluid isn't drained, they suffocate. You can get the bird euthanaised. Or the last option is a hysterectomy. Obviously the last one isn't an option if the birds are there purely to lay eggs as she'd never lay again after that. If the bird is a pet, some people are happy to have the surgery because it saves their beloved pet. Chances are it's not something that will turn into EP, but thought I should mention it just in case.
Jane