You won't be able to see anything so late now; it should be completely dark. Candling days are 7, 14, and before lockdown. You could candle now and it should be completely dark with an air cell at the top. If it's clear or has a line growing around the diameter, it's infertile or dead. Have you lockdowned yet??? By the way, what kind of eggs are you incubating?
Oh, okay. They're duck eggs. I was thinking they were chicken eggs! What breed? Lockdown is when you raise the humidity to about 65% and do not open the incubator. It starts on about the 25 day depending on the breed. About candling: you could've candled on days 7 and 14, but it's okay. You'll still be able to see any movement in the eggs most likely.