You might want to talk to your doctor, just to be on the safe side. However, at the same time my daughters will know something, but when asked to do it for someone else (or even us) they pretend not to. Both of my girls are older, but they both have went through this phase from the time they were able to do things and haven't stopped. I remember a time when our oldest started walking, when my mom got home (she lives with my husband & I), we tried to get Josephine to walk and she wouldn't. Not one minute after my mom went to her room she started to walk again!! It seems that whenever we want them to "perform" they don't want to - unless they are born natural performers I guess. At the same time, because she was adopted you may not have received all the information about her living conditions/environment during those 5 months (and you may have) and some of that may have impacted not only her speech development (or understanding of the English language) but also her motor skills development as well. A doctor should be able to help put some of the anxiety to rest. It is entirely possible that it is completely normal though - especially if you have seen, or had, her do it before. Some things just take time.
Our youngest was probably 2 1/2 before she started talking really clearly. We have a friend who is a speech pathologist and she told us that if it continued after the age of three to call the place she works and set up a consultation. She heard her talk, and seemed to think she would adjust - and she has. It seemed to happen suddenly over night.