What Causes A Missed Period After Taking Duphaston Despite Typically Getting My Period Within 4 Days After Completing The Medication?

Hi! I took Duphaston from October 2-11, but no period until now. I took pregnancy test but result is negative. What might be the reason? I am currently undergoing fertility work up. Usually after taking 10 days Duphaston, my period will come after 4 days.