I don't hatch guinea but I do chickens and occasionally turkey. The "ideal" hatch date is just a guideline. For different reasons eggs can hatch a day or even two early or late. Heredity, humidity, how and how log the eggs were stored or just difference in individual eggs eggs even if laid by the same hen can cause them to hatch at different times even if incubated together. A big factor can be average incubation temperature. If it is a bit low they can be late, high and they can be early. My chicken eggs often hatch a day or more early, whether under a broody hen or in my incubator. So don't get too tied up with that 28 days for guinea eggs.
I don't know what is going on with your eggs. With them all being late it might be that your incubating temperature is a bit low. Some thermometers are not that accurate and incubator factory pre-sets are often off a bit. Give your eggs another couple of days before you panic. Let us know how it goes either way.