Poking around with google search.

Assuming all seals and gaskets are working correctly and the correct transmission fluid used,
-check the transmission fluid level (even if you filled it to the maximum line. After a rebuild and initial test drive the level can drop because the fluid is filling spaces not normally emptied when flushing the transmission)
-shift lever/linkage adjusted properly?
-any electrical or vacuum connections to transmission?

take care,
ChrisS

note: I've only worked with manual transmissions. So, you might have to do your own google searches.