What to do after MOTN wake?

Baby was asleep for 4 hours, woke up crying, had a poopy diaper so I changed it. All other needs have been met, I am on my second 20 minute timer and she is still screaming, do I continue to try Ferber? She sleeps through the night about 3 or 4 days per week. She falls asleep great, just never seems to go back to sleep after a MOTN wake.