When you say you completed ALL, does that include the Challenges too? Does it say you're literally only missing ONE item to be collected or one LEVEL of the Achievement?
Each Achievement has 8 levels & usually players also need to collect the said item from the Challenges as well.
EDIT: Took a look at the Easter 2013 WIKI page. There's a Repeatable mime containing 18 Eggs. If everything is done, then you need to do the Hen Coop Repeatable mine until you have enough Eggs to complete the Achievement.